SECTION 3.00
STREETS

3.01 GENERAL

The latest revision of the NCDOT "Standard Specifications for Roads and Structures" shall apply unless otherwise specified herein. The following substitutions shall be read into the aforementioned specifications:

"State" or "Commission" shall be replaced by "Town of Cary".

"Resident Engineer" shall be replaced by "Director of Development Services" or their authorized representative.

"Sampling and testing by Commission" shall be replaced by the words "sampling and testing by the Town or its duly authorized testing agent".

"Inspection by Commission" shall be replaced by "Inspection by Town or its duly authorized representative".

3.02 DESIGN CRITERIA

All streets (private and public) shall be designed and constructed to Town Standards unless NCDOT Standards are applicable. NCDOT Standards shall be used on all existing state roads, extensions of existing state roads, or roads to be maintained by the state. Street design is based on criteria dictated by the street classification, design speed, surrounding terrain, and traffic volumes. Private streets will be permitted only for commercial uses.

A. Street Classifications

Streets are classified according to the nature of the traffic they serve, and are networked to provide a balance between access and mobility.

All streets shall conform to the Town of Cary Thoroughfare Plan when applicable or shall be designed and located in proper relation to existing streets and environment. Collector streets and arterials shall be as direct as possible, but also be consistent with topography and preserve developed properties and community values.

All streets shall be networked to provide safe and efficient access to all properties. No properties shall be landlocked or excessively removed from the major street facilities.

1. Local Streets

Local streets provide the highest degree of access and the least mobility. They are generally associated with residential areas and permit direct access to abutting land. Local streets shall be designed to discourage high speed traffic and minimize excessive cuts, and fills.

• ALLEYS

Alleys are permitted only in the Core Neighborhoods Overlay District and within Neo-Traditional Planned Unit Development designs. The purpose of an alley is to provide utility and vehicular access to new residential developments that cannot be adequately served by existing streets. All alleys are to be privately maintained with measures to ensure the travelway is not obstructed in any manner, including parking. A pavement design shall be submitted to and approved by the DDS-Development Review Division for an alley that will be used for public services (e.g. sanitation, fire protection). The Town shall not be responsible for damage to the pavement structure due to its use of the alley in providing public services.

• CUL-DE-SACS (Residential and Non-Residential)

Cul-de-sacs serve either abutting residential or non-residential land uses and terminate in a turnaround. Additional cul-de-sacs shall not intersect onto this street, which shall not exceed 900 feet in length.

• LOOP STREET

Loop streets serve abutting residential land uses, terminate on the same street from which they originate, and shall not exceed 1500 feet in length. Short residential cul-de-sac streets may intersect onto this street.

• RESIDENTIAL LOCAL STREET

This is a street whose primary function is to serve an immediately abutting residential land use. Traffic volumes from other intersecting streets shall not exceed the traffic volumes generated by the land use abutting the street. Primary design concerns focus on fostering a safe and pleasant environment for the residential community, with convenience to the motorist secondary. Particular emphasis is placed on providing adequate stopping sight distance in order to protect pedestrians.

• NON - RESIDENTIAL LOCAL STREET

This is a street which serves traffic generated by a non-residential abutting land use. Design shall accommodate the vehicle type expected to use the facility which is typically larger than a passenger car.

2. Collector Streets

Collector Streets provide both land access and mobility within neighborhoods and commercial areas. These are streets which penetrate various land use classifications. Their primary function is traffic service, collecting traffic from intersecting streets and funneling it to major thoroughfares.

• RESIDENTIAL COLLECTOR STREET

A residential collector street shall be provided when either traffic or access conditions dictate. Traffic conditions apply when the roadway, under immediate or ultimate buildout conditions, collects traffic from more than 100 dwelling units, or accounts for traffic volumes in excess of 1500 ADT. Access conditions apply when the extent of a development so isolates the remote units of that development from a Thoroughfare, that access by emergency or service vehicles can be deemed unsafe or uneconomic. Access conditions shall also apply where it is deemed reasonable and feasible to interconnect abutting neighborhoods.

• NON - RESIDENTIAL COLLECTOR STREET

A non-residential collector street shall be provided when either traffic or access conditions dictate. Traffic conditions apply when the roadway, under immediate or ultimate buildout conditions, collects traffic volumes in excess of 4000 ADT, or contains a commercial area of 20 or more acres. Access conditions apply when the extent of a development so isolates the remote units of that development from a Thoroughfare, that access by emergency or service vehicles can be deemed unsafe or uneconomic. Access conditions shall also apply where it is deemed reasonable and feasible to interconnect abutting development.

3. Arterial Streets

Arterial streets provide a high degree of mobility and limited access. Arterial streets can either be thoroughfares or highways upon the nature of the traffic. Controlled access highways tend to serve the through or bypass traffic of an urbanized area, while thoroughfares provide intracommunity travel within the urbanized area.

• THOROUGHFARE

Thoroughfares serve as a primary traffic artery of the urban area, serving the major centers of activity and carrying traffic between such centers at moderate speeds. Access to abutting property may be provided, but the primary function is to carry traffic originating from non-abutting areas.

• LIMITED ACCESS THOROUGHFARE

Typical Width shall be required as specified on the Town Thoroughfare Plan

This is an urban major thoroughfare whose sole function is to carry large volumes of traffic safely and expediently through the urban area. Access onto the facility is controlled to occur only at intersections with major streets or in some cases limited to right-in/right-out access as approved by the Town. Such intersections are spaced at intervals which promote traffic progression with the absolute minimal delays.

• CONTROLLED ACCESS HIGHWAY

Typical Width shall be required by Federal Highway Administration and AASHTO. This is a highway especially designed for through traffic, and to which owners of abutting property, shall have only a controlled right or easement of access.

B. Design Speed

Design speed is the maximum safe speed that can be obtained on a street when conditions are favorable enough for the design features of the highway to control. The design speed chosen for the design of a street should be logical with respect to topography, the adjacent land use, and the classification of the street. Every effort should be made to use as high a design speed as practical, under economic, environmental, aesthetic, and social constraints, to attain a desirable level of safety, mobility, and efficiency.

Once selected, all pertinent features of a street, such as curvature, superelevation, and sight distance, should be related to the design speed.

Local street design speeds range between 25 mph and 40 mph depending upon the available right-of-way, terrain, adjacent development, and other area controls. In particular, Cul-de-sacs and Loop streets are to have a minimum design speed of 25 mph, Residential streets are to have a minimum design speed of 30 mph, Non-Residential streets are to have a minimum design speed of 40 mph.

All Collector streets are to have a minimum design speed of 40 mph.

Arterial street design speeds range from 40 mph to 60 mph. In particular, minor arterial streets such as thoroughfares are designed for 50 mph. Major arterials, such as Limited Access Thoroughfares and Controlled Access Highways, shall meet design speeds in accordance with the guidelines in AASHTO’s Policy on Geometric Design of Highways and Streets.

C. Traffic Composition

The physical characteristics of variously sized vehicles have a direct impact on geometric design. Vehicles are classified into three main categories: passenger cars, trucks, and buses/recreational vehicles, as well as fifteen subsets as listed in AASHTO’s Policy on Geometric Design of Highways and Streets. Streets shall be designed such that the turning paths of these vehicles do not interfere with the physical constraints or other traffic of the street. The applicable category of vehicle to design is based upon the classification of the street.

Local streets can be subject to both passenger cars and trucks. In particular, Cul-de-Sacs and Loop streets are to be designed for passenger cars, Residential streets shall accommodate single-unit trucks, and Non-residential streets shall accommodate the type of vehicle prevalent to their access purpose.

Collector streets are to be designed to accommodate single-unit trucks.

Arterial streets can be subject to a wide range of design vehicles. Minor arterial streets like thoroughfares are to be designed for intermediate semi trailers. Major arterials, such as Limited Access Thoroughfares and Controlled Access Highways typically meet the design needs of vehicles specified by NCDOT.

3.03 TYPICAL CROSS SECTION ELEMENTS

A. Element Definitions

The elements which compose the cross section of a street should take into account the classification, design speed, traffic volume, traffic composition, and terrain of that street. The components which make up the cross section are the pavement type and cross slope, the widths for lanes, shoulders, curbs, sidewalks, and if necessary the median. All of these components lend themselves together into determining the required right-of-way width.

Listed below are definitions of each of the elements within a cross section.

1. Right of Way

See Section 1.00 "Definitions and Abbreviations".

Right of Way shall be sufficient to accommodate the required roadway section, shoulder section, and median section where applicable.

No fixed or non-breakaway objects, as defined by NCDOT, shall be permitted within the limits of the right of way.

2. Pavement Design and Cross Slope

A pavement design will be required for all collector streets and arterials. A pavement design shall also be required for all streets located within areas with Triassic soils (as shown on Figure 3). The pavement design shall be in accordance with these specifications. The pavement design and traffic analysis shall be signed and sealed by a NCPE. Pavement design shall be based on subgrade conditions, a 20 year design life and projected traffic loading. Subgrade conditions shall be based upon corrected soaked CBR values at 0.1 inch penetration per ASTM D1883. Soil samples used for these CBR tests shall be obtained at intervals not greater than 500 feet. Should an NCPE with expertise in geotechnical engineering certify that the soil in question is of the same type with similar engineering properties this spacing may be increased to a 700 foot maximum spacing. Boring logs and scaled drawings designating boring locations with CBR tests and other pertinent data shall accompany the pavement design.

The pavement design for areas with Triassic soils shall use the soaked CBR value for the pavement design. Typically, a subdivision will require 2 to 3 soil samples as a part of the pavement design. Larger subdivisions, greater than 100 lots, may require additional soil samples at the discretion of the DDS - Development Review Director.

Approved pavement design methods include those most current as proposed by NCDOT, AASHTO and the Asphalt Institute MS 1 document.

The AASHTO method will require use of a terminal serviceability index of 2.0 for collectors and 2.5 for thoroughfares, S_o = 0.49 for flexible pavement or S_o = 0.39 for rigid pavements, and a reliability of 98 percent for thoroughfares and 95 percent for collectors.

Rigid pavement design shall follow the most current AASHTO Method or the Portland Cement Association Method.

All streets maintained by the NCDOT must receive approval of the pavement design from the NCDOT prior to the placement of curb and gutter or pavement material.

Normal crown for the pavement section shall be 1/4" per foot.

3. Lane Widths

All collectors and thoroughfares shall be marked in accordance with the latest revisions of the MUTCD unless otherwise approved by the Town Engineer. This shall be noted on roadway and subdivision plans as a requirement of the Developer and shall be completed prior to issuance of a Certificate of Occupancy for the development or final acceptance of the roadway by the Town.

The Town of Cary considers bicycle traffic an important mode of transportation to be incorporated into thoroughfare and collector street design. Details should be resolved through the Town Engineer during design.

4. Shoulder Sections

Shoulders shall be sufficient to permit the adequate installation and maintenance of sidewalks and utilities, as well as provide sufficient clear distance as defined by NCDOT.

Shoulder sections without sidewalk shall be 10 feet wide on all streets with a cross section of 35 feet and greater.

Shoulder sections without curb and gutter must be a minimum of 6 feet wide.

5. Curb and Gutter

Curb and gutter shall be required on all streets with the following exceptions:

Residential cul-de-sacs in a Planned Unit Development;

Residential loop streets with low traffic volumes in a Planned Unit Development;

Streets within the Reservoir Watershed Protection District.

Streets designed without curb and gutter must meet all of the following requirements and be approved by the Town Council:

a. 50 feet of right of way;

b. 5% maximum and 0.5% minimum vertical grade;

c. Swales shall carry the 10 year storm in a non-erosive manner;

d. Driveway pipes shall pass the 10 year storm;

e. Driveway pipes shall have flared end sections or headwalls on both ends.

On all public streets, median curb shall be standard 18 inch mountable curb, and all other curb and gutter shall be standard 30 inch. No valley curb shall be used on public streets.

A minimum 5 foot section of curb and gutter shall remain when removing curb for the installation of a driveway, street turnout or repair of curb and gutter. When less than 5 feet of the curb remains, the curb shall be removed to the next joint.

6. Sidewalk

Sidewalks shall be constructed within the street right of way in accordance with Town Standards and Town policy. Sidewalks shall be installed at the time of roadway construction or widening unless otherwise approved by the Town Council. Sidewalk shall be required as follows:

• Thoroughfares, all collector streets and non-residential cul-de-sacs: sidewalk on both sides.
  
• Residential and non-residential local: Sidewalk on one side.
  
• Loop, and residential Cul-de-sac streets: No sidewalk unless either street is within one-half mile linear traverse of a greenway, park, shopping area, or the street is within 1.5 mile linear traverse of a school, in which case sidewalk on one side will be required.

The minimum thickness of a sidewalk shall be 4 inches. A 6 inch depth is required at locations where a driveway crosses a sidewalk, at street intersections (along the length of radius curb returns), and in the handicap ramps. Sidewalks shall have a uniform slope toward the roadway of not less than 1/4 inch per foot nor greater than 1/2 inch per foot. The utility strip between the sidewalk and the back of curb shall not be less than 1/4 inch per foot nor greater than 1/2 inch per foot toward the roadway.

Sidewalks shall typically be a minimum distance of five (5) feet off of the back of curb with a minimum width of five (5) feet.

Where sidewalks and/or greenways intersect any section of curb and gutter, a wheelchair ramp shall be installed.

7. Median Sections

Raised median sections shall be a minimum of 16 feet wide measured from the back of curb to the back of curb to provide 12 feet for left-turning vehicles and 4 feet for the placement of signs and separation of traffic at median openings. It is desirable to have continuous median sections on thoroughfare roadways. In no case shall plantings within a median obstruct required sight distance.

Medians shall have sufficient crown prefer 1/2"/Ft to promote drainage off the median, but shall never be to a cross slope in which sight distance is obstructed.

8. Asphalt Paths

Location of asphalt paths shall be in keeping with the Greenways Master Plan. In situations where asphalt paths are proposed to run parallel with roadways they shall be offset a minimum of 12’ from the back of curb. Asphalt paths will only be permitted parallel to roadways where there are limited number of driveway and street crossings. Asphalt paths shall be 1 1/2" I-2 underlain by 4" CABC. Width and the inclusion of additional pavement design features shall be site specific.

B. Street Classification Elements

1a. ALLEY (One Way)

Right-of-Way : 20’
Cross Slope: 1/4" per foot
Travel width : 12’
Shoulder width : 4’ (each side)
Curb & Gutter : N/A
Sidewalk : None

1b. ALLEY (Two Way)

Right-of-Way : 30’
Cross Slope: 1/4" per foot
Travel width : 16’
Shoulder width : 7’ (each side)
Curb & Gutter : N/A
Sidewalk : None

2. RESIDENTIAL CUL-DE-SAC

Right-of-Way : 55’
Pavement : 1/4" per foot
Roadway section : 27’ (Radius 45’)
Shoulder width : As required
Curb & Gutter : 2.5’
Sidewalk : One side (if required)

3. NON-RESIDENTIAL CUL-DE-SAC

Right-of-Way : 60’
Pavement : 1/4" per foot
Roadway section : 35’ (Radius 50’)
Shoulder width : As required
Curb & Gutter : 2.5’
Sidewalk : Both sides

A cul-de-sac radius shall be a minimum of 45 feet, measured from back of curb to back of curb. The standard maximum length for a cul-de-sac shall be 900 feet. The length may be varied by the Town Council depending upon the density within the subdivision. The recommendation for a variance shall consider the development density, land configuration, as well as all safety concerns. The length of a cul-de-sac shall be measured from the last point of alternate access. No median shall be allowed in a 45 foot radius cul-de-sac. A median may be permitted where the cul-de-sac radius is increased and it can be demonstrated that all emergency vehicles can be readily accommodated.

4. LOOP STREET

Right-of-Way : 50’
Pavement : 1/4" per foot
Roadway section : 27’
Shoulder width : 11.5’
Curb & Gutter : 2.5’
Sidewalk : one side (if required)

5. RESIDENTIAL LOCAL STREET

Right-of-Way : 50’
Pavement : 1/4" per foot
Roadway section : 27’
Shoulder width : 11.5’
Curb & Gutter : 2.5’
Sidewalk : one side

6. NON-RESIDENTIAL LOCAL STREET

Right-of-Way : 65’
Pavement : 1/4" per foot
Roadway section : 41’
Shoulder width : 12.5’
Curb & Gutter : 2.5’
Sidewalk : one side

7. COLLECTOR STREET (Residential and Non-Residential)

Right-of-Way : 60’
Pavement : 1/4" per foot
Roadway section : 35’
Shoulder width : 12.5’
Curb & Gutter : 2.5’
Sidewalk : both sides

8. THOROUGHFARE

Right-of-Way : As listed on Thoroughfare Plan
Pavement : 1/4" per foot
Roadway section : As listed on Thoroughfare Plan
Shoulder width : As required
Curb & Gutter : 2.5’ (1.5’ mountable curb along median)
Sidewalk : both sides
Medians: As required

9. LIMITED ACCESS THOROUGHFARE

Elements as required by the North Carolina Department of Transportation, and/or the Federal Highway Administration.

10. CONTROLLED ACCESS HIGHWAY

Elements as required by the North Carolina Department of Transportation, and/or the Federal Highway Administration.

3.04 GENERAL DESIGN ELEMENTS

A. Horizontal Alignment Controls

1. Superelevation

The rate of superelevation is controlled by several factors : climate (amount of snow and rain), terrain (rolling or level), location (urban or rural), and traffic composition (i.e. slow moving traffic). Given these factors it is concluded that no single maximum superelevation rate is universally applicable.

Generally it should be noted that superelevation rates of 0.04 and 0.06 are applicable to urban design, and superelevation may be omitted on low speed urban streets. The maximum superelevation rates for each classification of roadway are listed in Figure 1. These rates take into consideration all of the factors listed above.

2. Tangent Lengths

A minimum tangent of 150 feet is required between reverse curves for minor arterials and collector streets. This tangent shall be extended as necessary to provide the minimum runoff lengths for the curves superelevation per AASHTO guidelines. A 100 foot minimum tangent distance will be required between reverse curves for all local streets, however, reverse curves may abut without any tangent distance on residential local streets. The minimum tangent length of an approaching intersection should be thirty (30) feet for residential streets. All intersections of streets classified as collector or greater shall have a tangent section not less than 100 feet approaching the intersection.

3. Spirals

Spirals are to be included in the design of Thoroughfares (minor arterials) and higher classified streets. The length of the spiral is to equal the length required for superelevation runoff. The length is to be based on the maximum relative gradients listed in AASHTO’s, "A Policy on Geometric Design of Highways and Streets".

4. Curves

Curves are to be designed to establish the proper relation between design speed, superelevation, and side friction (developed between the road and tires). The maximum degree (or minimum radius) of curvature is a limiting value of curvature for a given design speed, superelevation rate, and side friction factors. Use of sharper curvature for a specified design speed would require superelevation or side friction factors beyond operational capacity and/or practicality. The maximum degree of curves for each roadway class is specified in Figure 1, and is a significant value in horizontal alignment control

Compound horizontal curves with the same direction of curvature shall have the radius of the flatter circular arc no more than one and one half times the radius of the sharper circular arc.

B. Vertical Alignment Controls

1. Grades

Street grades shall be established with respect to existing topography to avoid excessive grading and the removal of existing trees and vegetation whenever practical.

The minimum grade allowed on any street shall be one-half of one percent. (1/2%).

The maximum grade allowed when approaching an intersection is five percent (5%) for the last 100 feet of pavement before the intersection.

2. Curves

Vertical curves affect the gradual change between grades of a vertical alignment. The curves should produce a design which is safe and comfortable for the driver, pleasing in appearance, and adequate for drainage. To meet these criteria, the vertical curves are designed to curve at rates specified in Figure 1. The product of the curvature rates and the algebraic difference in adjoining grades is the basis for the minimum length of curves, unless this product is less than the minimum lengths indicated in Figure 1, in which case the values listed in Figure 1 are to be used.

Specifically, the design of vertical curves should focus on the particular concerns of both the crest and sag curves. The design of crest vertical curves should focus on providing sufficient sight distance. Crest vertical curves which are too abrupt in their curvature can impede the line of sight to drivers and thereby limit sight distance. The design of sag vertical curves should focus on rider comfort and drainage. Sag vertical curves which are too abrupt in their curvature can magnify the effects of the gravitational and vertical centrifugal forces acting on a rider. Sag curves which are too gradual can create substandard drainage conditions within the curve. The rate of vertical curvature is to insure that a minimum grade of 0.003 ft./ft. is obtained within a 50’ distance from the level point. In addition, sag vertical curves in cut situations should be avoided to prevent potential problems associated with the ponding of water.

The vertical curve controls found in Figure 1 shall also be followed at all times. The table is located at the end of this chapter.

The Town Engineer may approve street lighting which exceeds the standard Town requirements for residential streets so as to reduce the length of sag vertical curves, provided the street lights are operational prior to the issuance of any Certificate of Occupancy on such street. The minimum allowable length of sag vertical curves, where a street light is in the sag, shall be 20A for residential local streets, and 15A for cul-de-sacs and loop roads.

C. Sight Distance

1. General

Sight distance shall mean the length of roadway visible to the driver traveling along the roadway or waiting to enter or cross the roadway. Sight distance provides motorists the opportunity, traveling at design speeds, to prevent their vehicles in various situations from striking unexpected objects. Stopping sight distance and intersection sight distance are discussed in the following sections. Other topics related to sight distance, such as passing sight distance and decision sight distance, will be reviewed under AASHTO guidelines when the applicable situation arises.

The Town shall review all proposed development plans and landscaping planting plans for compliance with sight distance requirements. All new development within the Town's ETJ shall meet these requirements.

Some objects located within sight distance areas may not significantly obstruct the required visibility of the driver. The driver may be able to see over, under or around some objects within sight distance areas. Objects that may be required within sight distance areas include fire hydrants, utility poles and traffic control devices which are located to minimize visual obstruction. Other objects 12 inches in diameter and smaller, such as tree trunks and sign posts, may be allowed within sight distance areas if located so as to not substantially restrict the driver's view. The Director of Development Services shall determine what objects, if any, may be located within sight distance areas. Trees greater than 12 inches in diameter and located in the right of way shall be evaluated in accordance with other applicable Town policies and requirements.

Some conditions may exist that prevent the attainment of desirable sight distance due to social, economic or environmental consideration. In such cases, the maximum practical sight distance shall be obtained. In addition, where desirable sight distance is not attained, additional measures such as warning signs, reduced speed zones and other traffic controls may be imposed. In all cases, unless otherwise provided by Code or granted an exemption from the Director of Development Services, the minimum provision of adequate stopping sight distance shall be required. Conditions existing within the Town's B-1 Business District prior to the adoption of the Town's "Sight Distance" ordinance revision, dated February 11, 1993, shall be exempt from these requirements, but shall, if practical, meet the requirements as a part of any site development or building alteration.

The Town shall remove sight distance obstructions located within Town right of way. The Town will notify the NCDOT of sight distance obstructions located within their right of way. The Town shall provide written notification to the owner(s) of private property on which a sight distance obstruction exists. The property owner(s) shall be responsible for the prompt removal of the obstruction on their property, and may be liable for any damage resulting from their failure to remove the obstruction.

2. Stopping Sight Distance

Stopping sight distance is the distance required of a driver to perceive, react, brake, and stop before reaching a conflicting object in its path. The required stopping sight distance is dependent upon the reaction time of the driver, the design speed of the vehicle, and the grade of the roadway. Stopping sight distances for both level and graded situations are listed below in Tables 3.1 and 3.2 respectively.

TABLE 3.1
Minimum Stopping Sight Distance
For Level Conditions and Wet Pavement

Source: AASHTO A policy on Geometric Design of Highways and Streets, 1990.

TABLE 3.2
Adjustment Factors for Stopping
Sight Distance on Grades

*Assumed speed is lower than the design speed since vehicles normally travel at a slower speed on an upgrade.

Source: AASHTO A policy on Geometric Design of Highways and Streets, 1990.

 

Stopping sight distance is measured in the horizontal (plan) and vertical (profile) planes. In both planes, a driver must be offered an unobstructed line of sight to the roadway in front of them.

The horizontal stopping sight distance is measured along the middle of the travel lane from the driver to the object. The horizontal line of sight (Figure 4) is a straight line connecting the driver’s eye, which is located in the middle of the travelway, with the object, which is also located in the middle of the travelway. If this line of sight is impeded by any obstructions, either the obstruction should be moved or the alignment adjusted.

The vertical stopping sight distance is measured along the middle of the travel lane from the driver to the object. The vertical line of sight (Figure 5) is a straight line connecting the driver’s eye, which is located 3.5 feet above the roadway surface, with the object, which is located 0.5 feet above the roadway surface. If this line of sight is impeded by any obstructions, either the obstruction should be moved or the alignment adjusted.

3. Intersection Sight Distance

Unlike stopping sight distance, which represents an absolute minimum applicable to all driving scenarios, intersection sight distance is only applicable at the intersection of two streets or the intersection of a street and driveway. At all intersections, there is a minor street or movement (whose approach is controlled by some device like a sign), and a major street or movement (whose approach may not be controlled). Intersection sight distance is the distance required of a driver on the minor street to react and safely cross or join the major street traffic, while not requiring the major street traffic to reduce its speed or alter its path.

In addition, intersection sight distance is based on conflicts with opposing vehicles rather than with objects located in the roadway.

Intersection sight distance is measured in the horizontal (plan) and vertical (profile) planes. In both situations, a driver must be offered an unobstructed line of sight to the roadway they wish to cross or join. The horizontal intersection sight distance is measured along the centerline of the major street between the drivers of the two opposing vehicles.

The horizontal line of sight (Figure 6) is a visual line connecting the driver’s eye and the approaching vehicle, both of which are in the center of the travelway. If this line of sight is impeded by any obstructions, either the obstruction should be moved or the alignment adjusted. The vertical stopping sight distance is measured along the centerline of the major street between the drivers of the two opposing vehicles. The vertical line of sight (Figure 7) is a visual line connecting the driver’s eye, which is located 3.5 feet above the roadway surface, with the approaching vehicle, which is located 4.25 feet above the roadway surface. If this line of sight is impeded by any obstructions, either the obstruction should be moved or the alignment adjusted.

The amount of sight distance required at an intersection depends on the type of traffic control at the intersection and the speeds of the vehicles.

a. Yield Sign Control

This type of design requires that the side street be posted with yield signs. The sight distance for the driver on the side street (minor movement) must be sufficient for the driver to observe a vehicle on the through street (major movement) approaching from either the left or the right and bring his/her vehicle to a stop prior to reaching the intersection as shown in Figure 8.

Where proper sight distance cannot be achieved for the driver on the side street, it may be necessary to have a posted speed reduction on the approach or to replace the yield sign with a stop sign. Adequate sight distance shall also be provided for safe departure from a stopped condition.

b. Stop Sign Control

Where traffic is required to stop, the driver of the stopped vehicle shall have adequate sight distance to cross or join the approaching traffic flow without adversely affecting the travel speed of the approaching traffic. There are three basic maneuvers that occur at stop controlled intersections:

1. To travel across the intersecting roadway by clearing traffic on both the left and the right of the crossing vehicle;

2. To turn left into the crossing roadway by first clearing the traffic on the left and then entering the traffic stream with vehicles from the right (this maneuver is similar in nature to that made by the median left turns off the through street); and
   
3. To turn right into the intersecting roadway by entering the traffic stream with vehicles from the left.

Where the through street is undivided, or divided with a median narrower than 20 feet, maneuvers I and II are treated as a single operation. Where the median can provide storage for the design vehicle (20 feet wide for a passenger car), maneuvers I and II may be considered in two separate phases of operation.

The measurement method for determining the sight line for left, right, and through movements from the side street is based on values listed in Table 3.3 and illustrated in Figure 9.

TABLE 3.3
Intersection Sight Distance for
Stop Sign and Traffic Signal Controlled Intersections (right on red movements)

Source: AASHTO, A Policy on the Geometric Design of Streets and Highways, 1990.

The measurement method for determining the sight line for left turns from the median lane of the through street is based on values listed in Table 3.4 and is illustrated in Figure 10. The values in Table 3.4 gives the minimum sight distance for movement across one, two, or three lanes.

TABLE 3.4
Minimum Sight Distance for
Left Turn from Through Street

Source: AASHTO, A Policy on the Geometric Design of Streets and Highways, 1990.

c. Traffic Signal Control

At intersections controlled by traffic signals, the minimum sight distance will be stopping sight distance (Tables 3.1 and 3.2) for all side street movements except for the right turn movement. The right turn movement shall have intersection sight distance to allow right turn on red, except where it is economically impractical due to existing major features such as permanent buildings and large, mature trees. The minimum sight distance for the right turn movement limited by existing major features shall be stopping sight distance. Where intersection sight distance cannot be achieved, right turn on red will be restricted.

D. Traffic Control

1. Traffic Signs, Pavement Markings, and Street Name Signs

All streets require traffic signs. All collectors and arterials require traffic signs and pavement markings. Traffic signs and pavement markings shall be marked in accordance with the latest revision of the MUTCD unless otherwise approved by the Town Engineer. A pavement marking plan showing description and placement of traffic signs, pavement markings, and specialty signs shall be submitted with all roadway and subdivision plans as a requirement of the Developer, and shall be done prior to issuance of a Certificate of Occupancy for the development or final acceptance of the roadway by the Town. The pavement markings for all streets, both public and private, shall be thermoplastic in accordance with NCDOT standards. Exception of the use of thermoplastic is granted in the case of private parking stalls.

Traffic Control and Street Name Signs within subdivisions which will be maintained by the Town shall be consistent with the MUTCD. All specialty traffic control and street name signs and posts must comply with Policy Statement Number 85 including all amendments as approved by the Town Council. Requests for specialty signs submitted to and approved by the Engineering Division.

2. Traffic Signals

The design, installation and construction of traffic signals shall meet the specifications put forth by NCDOT’s Traffic Engineering Branch in the latest version of their manual "Traffic Signal Specifications". Special attention shall be given to the areas of these specifications regarding metal poles and preemption control. All control equipment shall be programmed.

• Metal Poles

Metal poles with mast arms shall be employed at intersections where utilities are underground. All other locations will be decided on a case by case basis. Where applicable, consideration should be given to standard designs of metal poles and mast arms and footings as approved by NCDOT.

• Preemption Control

Emergency vehicle-initiated preemption of traffic signals on the State Highway System must be approved by NCDOT. Official first response emergency vehicles that utilize sirens and red flashing lights to provide services to the public which prevent loss of life and property are the only eligible vehicles. "Official" vehicles are fire-fighting and emergency medical services vehicles owned, operated and maintained by the emergency response agency or authority.

Police vehicles may have access to the preemption system on state maintained roadways only after approval by NCDOT. All preemption equipment must be on the Traffic Signal Equipment Qualified Products List (QPL). The Traffic Engineering and Safety Systems Branch manages this QPL and is responsible for assuring the equipment is reliable, cost effective and compatible with NCDOT and agency requirements.

3. Traffic Control Devices

The Town will not allow the obstruction of any public street, private street or fire lane unless otherwise stipulated by the Town Council. This requirement is in accordance with the 1992 edition of the North Carolina State Fire Prevention Code. The reference to an "obstruction" shall include parking, speed bumps or any other device which may obstruct the free passage of emergency vehicles.

All traffic control devices allowed outside the above restrictions must be shown and approved as a part of a site plan prior to installation and must be in conformance with the Specifications. The traffic control devices and all related signs and pavement markings shall be maintained by the Owner. Traffic control devices may include rumble strips, raised pavement markers, pavement undulations (speed humps), or speed bumps.

• Rumble Strip

An irregular surface established to draw the attention of motorists and alert them of potential hazards or conflicts. The material used as a part of a rumble strip may be cobblestone, stamped concrete, brick or rough surface asphalt. A rumble strip may not vary more than one (1) inch in height from the pavement elevation. All rumble strips must be located outside any public right of way.

• Raised Pavement Markers

Raised pavement markers may be used to create an irregular surface to draw the attention of motorists and to alert them of potential hazards or conflicts. The markers must be made of a flexible and durable solid material designed to support vehicular traffic. The placement of markers may be staggered in a manner as approved by the Director of Development Services. All raised pavement markers shall have a maximum height of 1 inch above the pavement surface. The minimum size of the marker shall be 4 inches by 4 inches. The markers must be located outside any public right of way. All raised pavement markers shall have cube-corner microprism reflectors visible from either direction of travel.

• Pavement Undulation (Speed Hump)

A pavement undulation is a gradual raised pavement surface which transverses the travelway. The surface material for a pavement undulation shall be the same as the adjacent travelway, typically asphalt.

A pavement undulation shall be parabolic with a maximum height of 3 inches. The undulation shall be tapered to the edge of the pavement or gutter line for the last 12 inches along each side. The minimum length of the pavement undulation shall be 12 feet.

All pavement undulations shall be clearly marked with pavement markings and warning signs for each direction of travel.

Pavement undulations must be located a minimum distance of 500 feet from a signalized intersection. The Town and/or NCDOT may require the removal of any speed undulation which causes traffic to back up onto a public street.

• Pedestrian Crossings

All locations which are designated for pedestrian traffic crossings shall be designated as a crosswalk with pavement markings and signage. The type and placement of signage and markings shall be consistent with MUTCD. Pedestrian crossings should be placed at traffic controlled approaches to intersections and included in the phasing. All pedestrian crossings must be approved by the Engineering Division prior to installation in public areas. Private areas are under the review of Development Review Division.

4. Lane Configuration

Tapers shall be used as necessary in street design. Approach tapers are used to shift lanes laterally. The following equations shall be used as applicable:

L = WS for posted speeds of 45 mph and greater;

L = WS²/60 for posted speeds of 40 mph or less;

Where: L = Length in feet

S = Speed in miles per hour

W = Lateral offsets in feet.

5. Construction Zone Traffic Control

See section 2.06, "Maintenance of Traffic".

E. Traffic Analyses

All traffic analyses shall be completed and bear the seal of a N.C.P.E. See section 16.1.19 of the Unified Development Ordinance for other requirements.

F. Structure Design

All public or private bridges shall be designed to withstand HS-20 highway loading unless otherwise approved by the Town Engineer and shall be properly signed and sealed by a NCPE.

3.05 SPECIFIC DESIGN ELEMENTS

A. Intersections

1. Alignment

Streets shall intersect each other at right angles whenever possible. The minimum desirable intersection angle is 80 degrees. At no time shall a street intersect any other street at less than 75 degrees.

2. Layout Configuration

Curb radii shall be designed to satisfy the turning radius of the predominant design vehicle using the roadway. Any intersections tying into a Residential Local Street shall have a minimum radius of 25 feet. Any intersection tying into a Non-Residential Local Street shall have a minimum radius of 30 feet. The minimum radius for Residential Collector Streets shall be 30 feet, while Non-Residential Collector Streets shall be 40 feet.

Turn bay tapers shall be straight line at a ratio of 15:1 for posted speeds of 45 miles per hour and more. The minimum turn bay taper allowed is 8:1 for posted speeds below 40 miles per hour. Symmetrical reverse curve tapers are recommended for non-thoroughfare streets as shown in the details. Storage lengths for the turn bays shall be calculated using an acceptable method.

Streets with medians shall be designated to allow for proper turning movements for a SU (single unit truck) design vehicle. AASHTO guidelines should be followed for the actual median design and median opening dimension.

3. Spacing

There shall be a minimum of 200 feet between centerlines of street jogs on collectors and arterials. Local streets shall not be offset less than 125 feet from their centerline.

Median breaks shall be provided to allow safe and efficient movement of traffic. The desirable spacing of median breaks shall be at 1000’ intervals, with the minimum allowable spacing to be at 500’ intervals.

Intersections of roadways controlled by a traffic signal should be spaced along roadways at the following intervals:

Arterial Streets 1/2 -1 mile
Collector Streets 1/4 mile
Local Streets 1000’

Four legged intersections not controlled by a traffic signal should be spaced along roadways at the following intervals:

Arterial Streets 1000’
Collector Streets 750’
Local Streets 300’

Three legged intersections not controlled by a traffic signal should be spaced along roadways at the following intervals:

Arterial Streets 800’
Collector Streets 500’
Local Streets 200’

B. Driveways

1. Non-Residential Driveways

Standard concrete driveway aprons shall be used when the ADT for the driveway is less than 300 vehicles.

Street type turnouts shall be used when the driveway ADT is greater than the above listed conditions or when access by larger trucks must be accommodated. A minimum radius of 30 feet shall be used on all street type turnouts. The maximum grade allowed when approaching an intersection is five percent (5%) for the last 30’ before the edge of the intersection.

Non-residential driveways that are unpaved shall have a minimum 20 foot paved surface strip measured from the back of the driveway apron.

All driveways shall have a minimum width travel lane of 25 feet. Any curb and gutter used will be in addition to the 25 foot minimum width.

Non-residential driveways with islands shall have a 16 foot entrance lane. A 16 foot exit lane shall be required when one exit lane is used, and a 24 foot exit shall be used for 2 exit lanes.

The number of street and driveway connections permitted serving a single property frontage or commercial development shall be the minimum deemed necessary by the Town Engineer for reasonable service to the property without undue impairment of safety, convenience, and utility of the roadway. Normally, not more than two driveways shall be permitted for any single property frontage.

The arrangement of driveways should be related to adjacent driveways and nearby street intersections. Driveways accessing local or collector type streets shall be at least 100 feet from the point of tangency of the radius curvature of the next intersecting street. Driveways serving traffic volumes in excess of 300 ADT or accessing thoroughfares shall be located a minimum of 250 feet from the point of tangency of the radius of curvature of the intersecting street.

Where two driveways are proposed along a single property frontage to facilitate operations, the minimum distance between the centerlines of the drives shall be 200 feet. The minimum distance between the centerlines of driveways into shopping centers or facilities generating in excess of 300 ADT shall be a minimum of 400 feet. Full access driveways open to signalization should be 1000’ apart. Driveways which access thoroughfares and serve more than 1500 ADT shall provide deceleration lanes in approach to the driveway.

2. Residential Driveways

Residential drives shall be 12 to 18 feet wide and shall conform to the Standard Detail for concrete driveway aprons.

Residential drives shall be located a minimum of 10 feet from the point of tangency of curb radii of street intersections.

Where driveway aprons are in sag locations, installation of a trench drain in the gutter line may be acceptable as a means to alleviate drainage problems. The Town shall not be responsible for the maintenance of residential driveways, except where the driveway adversely affects the maintenance, performance, or structural integrity of the abutting street, utility strip, or sidewalk.

C. Parking Lots

Parking lots shall be designed to provide safe maneuverability of vehicles. A minimum parking stall dimension of 9 by 18 feet shall be provided. Handicap parking spaces shall be a minimum of 8 feet wide with a 5 foot lane adjacent to the space properly marked with signage in accordance with the N.C. Building Code. All parking facilities shall have dimensions as outlined on Figure 2 and the Town Parking Ordinance. The measurement of said dimensions shall be to the front edge of curb and gutter at the edge of pavement, unless a curb stop is employed in which case that will serve as the end of measurement.

At locations where sidewalk abuts a parking bay which is 18 feet deep, the sidewalk shall be a minimum width of 6 feet. In parking lots where end islands are required, reference to Standard Detail is recommended.

A minimum pavement structure consisting of 8 inches of ABC stone and 2 inches of I-2 asphalt shall be used in the travel aisles and stalls on parking facilities for multi-family (excluding duplex and triplex) developments, and non-residential developments. Access drives for these facilities shall also meet this minimum pavement standard.

All paved parking facilities shall be striped in accordance with the MUTCD with 4 inch white lines.

On all parking lot facilities, cut and fill slopes shall not begin immediately at the back of curb, instead a minimum 4’ shoulder is required behind the back of curb.

D. Fire Lane

Fire Lanes shall be installed and inspected in accordance with the public street requirements and the fire code. The Fire Official shall have the authority to designate fire lanes as deemed necessary for Fire Department access. The requirements for installation of a fire lane shall be subject to the fire code.

Fire lanes shall be a minimum width of 20 feet and shall be properly marked and signed to designate the access as a "fire lane" as specified by the Fire Marshal. The surface of the fire lane shall be paved with a minimum of 8 inches of ABC stone and 2 inches of I-2 asphalt.

All fire lanes shall be marked in accordance with one of the following requirements:

Continuously marked with thermoplastic yellow striping along the fire lane with "No Parking Fire Lane" printed with minimum 8 inch high letters at 40 foot intervals or as directed by the Fire Marshal;

Continuously marked with thermoplastic yellow curb with "No Parking - Fire Lane" along the fire lane with "No Parking - Fire Lane" printed with minimum 8 inch high letters at 40 foot intervals or as directed by the Fire Marshal:

The installation of the MUTCD standard sign showing "No Parking - Fire Lane" placed at each end of the fire lane and at 50 foot intervals with arrows on the signs or a continuously painted yellow strip along the designated fire lane.

3.06 MATERIALS

Portland cement concrete for curb and gutter, driveways, and sidewalks shall have a minimum 28 day compressive strength of 3000 psi, a non-vibrated slump between 2.5 and 4 inches, a minimum cement content of 564 pounds per cubic yards, an air entrainment of between 5 and 7%, and a maximum water-cement ratio of 0.532. (Also see Sec. 2.07 "Concrete")

Joint filler shall be a non-extruding joint material conforming to ASTM C1751.

Concrete Curing Agents shall be free from any impurities which may be detrimental to the concrete and meet Section 1026 of the NCDOT Standards.

Aggregate for portland cement concrete shall meet the requirements for fine and course aggregate of Section 1014 of the NCDOT Standards.

Portland Cement and admixtures shall meet the requirements of Section 1000 of the NCDOT Standards.

Water for mixing or curing the concrete shall be free from injurious amounts of oil, salt acid, or other products injurious to the finished product.

Aggregate Base Course shall consist of coarse aggregate produced in accordance with Section 910 of the NCDOT Standards for either Type A, B, or C aggregate.

Bituminous Surface Course, Type I-1 and I-2, shall consist of a mixture of coarse and fine aggregates, asphalt cement, and shall meet the requirements in Section 645 of the NCDOT Standards.

Bituminous Concrete Base Course, Type HB, shall conform to the general, material, and construction specifications of Section 610 and Section 630 of the NCDOT Standards.

Bituminous Concrete Binder Course, Type H, shall conform to the general, material, and construction specifications of Section 610 and Section 640 of the NCDOT Standards.

Tack Coat shall be asphalt or asphalt cement and shall meet the general, material, and construction specifications of Section 605 of NCDOT Standards.

Concrete Pavement shall meet the general, material, and construction specifications of Section 700 of the NCDOT Standards.

Concrete Pavers may be used on privately maintained streets in accordance with Standard Detail 3.04. The Town will not maintain decorative type paved street surfaces such as pavers or imprinted designs within public right of way.

Geotextile Fabric may be used to stabilize roadways, subgrades, slopes, and for other uses as necessary. The material must be approved by the Town Engineer prior to installation. Areas stabilized with fabric shall be indicated on "as-built" drawings with the manufacturer name and type fabric indicated.

3.07 CONSTRUCTION AND INSPECTION

A. Streets

No base material shall be placed on a roadway until the storm sewer, subgrade, utilities, and all appurtenances have been inspected and approved by the Inspector.

The Inspector may require field density testing of the subgrade soils by a certified testing firm. The firm shall perform sufficient Proctors to evaluate the compaction characteristics of various soils used in the roadbed. The Inspector may also require field density testing of the ABC used and an asphalt mix formula before either is inspected or approved.

The subgrade shall be compacted as described in Section 2.05 Earthwork. Inspection of the subgrade prior to placement of base course, and inspection of the base course prior to placement of asphalt shall be performed by proofrolling and/or field density testing at the direction of the Inspector.

B. Curb and Gutter, and Sidewalks

No concrete shall be placed until the forms and subgrades have been approved by the Inspector.

The surface of sidewalks shall be finished to grade and cross section with a float, trowelled smooth and finished with a broom. Sidewalks must be satisfactorily installed before issuance of CO.

• Subgrade shall be excavated to the required depth, and shaped to the proper cross-section. Where tree roots are encountered, they shall be removed to a depth of 1 foot for the full width of the excavation. The subgrade shall be stable and thoroughly compacted.
  
• Forms shall be set and maintained true to the required lines, grades, and dimensions. Forms shall be constructed with material of such strength and rigidity to prevent any appreciable deflection between supports. Straight forms shall be within a tolerance of 1/2 inch in 10 feet from a true line horizontally or vertically. Forms shall be thoroughly cleaned of all dirt, mortar and foreign material before being used. All inside form surfaces shall be thoroughly coated with commercial quality form oil.
  
• Grooved Contraction Joints shall be cut to a depth equal to at least 1/3 of the total slab thickness. The joint shall be no less than 1/8 inch in width and cut at intervals equal to the width of the sidewalk.
  
• Expansion Joints shall be a 1/2 inch joint filled with joint filler and placed between all rigid objects and placed no farther than 50 feet apart for sidewalks and curb and gutter, extending the full depth of the concrete with top of the filler 1/2 inch below the finished surface.

FIGURE 1

A - Algebraic difference in grades.

‡ - The Town Engineer may approve street lighting which exceeds the standard Town requirements for residential streets so as to reduce the length of sag vertical curves, provided the street lights are operational prior to the issuance of any Certificate of Occupancy on such street. The minimum allowable length of sag vertical curves, where a street light is in the sag, shall be 20A for residential local streets, and 15A for cul-de-sacs and loop roads.

* - Design speed shall be at least 5 mph greater than posted speed, except for cul-de-sacs, and Loop Roads, in which case the design speed is the posted speed.

+ - Radius will be measured to the centerline of the inside lane on Thoroughfare design.

** - The superelevation tables found in "A Policy on Design of Urban Highways and Arterial Streets - 1973" published by the American Association of State Highway Officials will be used for determining the actual "e" various radii.

*** - On roadways less than 500 feet in length, grades may be increased 1%.

FIGURE 2
PARKING LOT STALL DIMENSIONS

DIMENSIONS FOR 90-DEGREE PARKING

DIMENSIONS FOR 60-DEGREE PARKING

DIMENSIONS FOR 45-DEGREE PARKING