Public streets and highways are designed to accommodate a variety of vehicles, up to and including semitrailer trucks. When private driveways and roadways will serve only passenger vehicles, it may be appropriate in some instances to use smaller dimensions. Nevertheless, be certain private roads are wide enough to allow passage of fire and emergency vehicles.
When designing street layouts in residential developments, there are a number of important additional considerations, including safety. The underlying concept of the shared street system is one of integration, with an emphasis on the community and the residential user. Pedestrians, children at play, bicyclists, parked cars, and moving cars all share the same street space.
RESIDENTIAL STREETS AND GUIDELINES FOR HEALTHY NEIGHBORHOODS
When designing street layouts, especially suburban streets and neighborhoods, it’s important to avoid directing major traffic circulation through residential developments. Consider the following four key design criteria for use within residential areas:
- Minimize the amount of internal travel while maximizing convenience of access to nonresidential activities
- In any residential street layout, provide a series of spaces in which the road pavement plays an integrated function, as opposed to the typical linear pavement that just facilitates automobile movement
- Residential streets are regularly used as play areas, so design them to reflect a pedestrian orientation rather than only to facilitate vehicular movement
- Set the radius of cul‐de‐sacs to the minimum required to accommodate emergency and maintenance vehicles, while considering alternative turnarounds
TRAFFIC CALMING IN RESIDENTIAL DEVELOPMENT
Traffic calming improves neighborhood livability and the pedestrian environment by reducing vehicle speeds and noise, visual impacts, and through‐traffic volumes. Traffic calming includes a variety of design techniques and traffic management programs:
- Vertical changes to the road (e.g., speed bumps and humps, raised intersections, etc.)
- Lateral changes in the road (e.g., chicanes, narrowing, traffic diverters, etc.)
- Traffic circles
- Small corner radii
- Gateway features
- Related streetscaping (street furniture, lighting, landscaping, etc.)
For streets of up to 750 vehicles per day, traffic and delay considerations do not necessitate the need for more than one traffic lane. A single‐lane configuration is sufficient if parking lanes are incorporated or passing places are provided. For example, the width of residential streets may be as narrow as 20 ft. with parking on one side. Through careful design, this can produce cost savings in grading, drainage, and street construction, and lessen the amount of impervious surfaces.
DESIGN CONCEPTS AND GUIDELINES FOR RESIDENTIAL SUBDIVISION PLANNING
A number of different design concepts may be implemented to help preserve the residential feel when applying street design guidelines for healthy neighborhoods. They can range from on-street parking and shared streets, to more nuts-and-bolts features like sufficient vehicle turning radii and fire vehicle access.
On‐street parking should be encouraged whenever possible, as it is a highly efficient form of parking, especially where higher housing densities and a compact neighborhood are desired. On‐street parking should be especially encouraged near the center of the neighborhood planning area, closest to the neighborhood focal point, services, transit stops, and so on. On‐street parking also provides an important physical and psychological buffer between pedestrians on the sidewalk and traffic. The commonly held perception that on‐street parking will create situations where children dart from behind a parked car into passing traffic is not borne out by actual experience.
The underlying concept of the shared street system is one of integration, with an emphasis on the community and the residential user. Pedestrians, children at play, bicyclists, parked cars, and moving cars all share the same street space. Even though it seems these uses conflict with one another, the physical design is such that drivers are placed in an inferior position. Through geometrical and physical changes in the street cross section and its physical appearance, motorists sense that they are intruding into a pedestrian zone and drive more slowly and cautiously. By redesigning the physical aspects of the street, the social and physical public domain of the pedestrian is reclaimed.
Alleys eliminate the demand for automobile access to the property from the front, reduce the need for curb cuts, and reduce the number of parking garages oriented to the street. Alleys also provide an excellent alternative for utility easements and service access. The typical alleyway right‐of‐way width is 15 to 20 ft., with pavement typically covering the full width of the right of way.
Sufficient Vehicle Turning Radii
The vehicle to be accommodated—the design vehicle—is an important control in the design of urban streets. The most important characteristic of the design vehicle, its turning radius, dictates intersection design. For urban streets, four design vehicles are defined in the American Association of State Highway and Transportation Officials (AASHTO) Green Book.
Vehicle turning radii, as established by AASHTO, are based on broad categories of vehicle types, each of which encompasses a wide variety of actual vehicles on the road. Information on passenger vehicle dimensions for parking design, provided below, provides a more detailed breakdown of vehicle types within the various categories. For off‐street design applications, such as parking and driveways, the more detailed versions are appropriate. For public streets, the AASHTO templates are definitive.
Design Vehicle Dimensions
|VEHICLE||LENGTH (L) (ft.‐in.)||WIDTH (W)||HEIGHT (H)||WHEELBASE (WB)||OVERHANG FRONT (OF)||OVERHANG REAR (OR)||GROSS WEIGHT (LB)|
|Composite passenger vehicle2||17‐2||6‐7||7‐6||9‐11||3‐0||4‐4||6300|
|Wheelchair lift van (personal use)||17‐8||6‐8||8‐0||11‐6||2‐6||3‐8||6000|
|Boat trailer||20‐0||8‐0||6‐0||See detail||3‐0||8‐0||4000|
|RV–conventional trailer||27‐0||7‐0||9‐0||See detail||3‐0||8‐0||4000|
|RV–fifth wheel (pickup‐based)||34‐0||8‐6||12‐0||8‐0||22‐0||12‐2||3500|
|Slide‐in pickup camper||18‐11||10‐0||7‐3||—||—||—||2900|
|Shuttle van (11 passengers)||20‐0||6‐6||6‐10||11‐6||3‐0||5‐6||11,000|
- Small car classes 5 through 7, per Parking Consultants Council (PCC).
- A composite passenger vehicle is a design vehicle that encompasses passenger cars, light trucks, vans, and sport/utility vehicles. It is the vehicle for which a parking facility should be designed.
Fire Apparatus Access
According to the U.S. General Services Administration, in its post on site circulation design, ground-level and aerial access for fire department vehicles is an important component of planning. Access for vehicles should be provided, and the local fire department should be consulted regarding the specific requirements for the following:
- surface material of the access roadway(s)
- minimum width of fire lane(s)
- minimum turning radius for the largest fire department apparatus
- weight of largest fire department apparatus
- minimum vertical clearance of largest fire department apparatus.
Buildings or portions of buildings exceeding 30 ft. in height from the lowest point of fire department vehicle access shall be provided with access roads capable of accommodating fire department aerial apparatus.
FURTHER READING ON VEHICULAR CIRCULATION CONSIDERATIONS
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