Design for the most vulnerable street user rather than the largest possible vehicle. While designs must account for the challenges that larger vehicles, especially emergency vehicles, may face, these infrequent challenges must not dominate the safety or comfort of a site for the majority of daily users. The selection of design vehicle influences the physical characteristics, safety, and operations of a roadway.
Discussion
The selection of a design vehicle impacts the ultimate design characteristics of that street.1 Before selecting a design vehicle, consider the ideal design given the overall context of the roadway, understanding how larger vehicles might flexibly operate within the proposed design.2
Curb radii designed to accommodate the largest possible vehicle at its highest possible speed degrade the pedestrian environment and result in longer crossing distances.3
Large emergency vehicles, such as fire trucks, have certain ideal dimensions for operation often tied to response times. Assume that emergency vehicles are permitted full use of the right-of-way in both directions, especially where tight curb radii may necessitate use of the opposite lane during a turn.4
Transit vehicles, such as articulated buses, benefit from the use of a larger effective turning radius, which is benefitted by bikeways and/or on-street parking.
Oversized trucks and other large vehicles may be restricted from certain corridors based on existing context, vulnerable street users, or impractical operational impacts. Reroute trucks to parallel routes where extensive reconstruction is not required to meet their needs.
Critical
The design vehicle is a frequent user of a given street and dictates the minimum required turning radius; a control vehicle is an infrequent large user. The design vehicle can turn using one incoming and one receiving lane; the control vehicle can turn using multiple lane spaces.
Adopt both a design vehicle and a control vehicle standard based on context-specific city street types. The design vehicle determines the design of elements such as turning radius and lane width. The control vehicle dictates how the design might accommodate a larger vehicle’s turning needs when using the whole intersection.5
BROOKLYN, NY
At this intersection, a frequent bus route requires a tight right turn. A recessed stop line paired with a “stop here” sign is used so the bus can complete its turn in one maneuver.
City transit buses must be able to turn on bus routes without resorting to a 3-point operation. Where a 3-point turn would be necessary, designers should consider removing parking spaces near the intersection or recessing the stop line on the receiving street.
Use “crawl” speeds, as opposed to design speed, when determining local street geometry factors associated with the design vehicle.6 Vehicles traveling at slower speeds have more flexibility and can make difficult turns that may be challenging or unsafe at higher speeds.
Controlling Turn Speeds and Recessed Stop Bars
Recommended
This truck is an example of the DL23.
Adopt a new design vehicle that is a frequent user of urban streets—the delivery truck (DL23). Package delivery trucks commonly travel on city streets, and have an inside turning radius of 22.5 feet and an outside turning radius of 29 feet.
All truck routes should be designed to permit the safe and effective operation of trucks. Designation of freight routes should be considered in coordination with mapping of primary bicycle, transit, and pedestrian corridors, as well as through the analysis of key access routes, bridge hazards, and industrial or commercial land uses. Pair truck route programming with enforcement to ensure that oversize vehicles are not diverting off-network.
Where truck routes intersect and frequent turns are made, install bollards at the intersection corner to help prevent injuries and fatalities from truck wheels overrunning the curb.7
The design vehicle types below should be considered in order to maintain property access while emphasizing pedestrian safety and low speeds.
The largest frequent user of urban streets is the DL-23.
Optional
On narrow commercial streets that require frequent loading and unloading, consider the application of a shared street design to avoid large turning radii or freight vehicles parking on sidewalks.
Where trucks or city buses are expected to frequently encroach on the centerline, consider using a dashed centerline at the potential conflict point at the intersection.
- A Policy on Geometric Design of Highways and Streets, (Washington, D.C.: AASHTO, 2011), Section 2-1. ↩︎
- Kendra K. Levine, Curb Radius and Injury Severity at Intersections (Berkeley: Institute of Transportation Studies Library, 2012). ↩︎
- Most state vehicle codes stipulate that drivers should turn right “as close as practical” to the right-hand curb or edge of the roadway. “Practical” is not defined and the code does not ban use of multiple lanes to complete a turn if needed. For example, see:
Illinois General Assembly, “625 ILCS 5/ Illinois Vehicle Code,” Illinois Compiled Statues (Springfield). ↩︎ - Curb extensions can improve emergency vehicle access by keeping the intersection clear of parked cars.
FHWA Safety Program, “Traffic Calming,” accessed June 3, 2013. http://safety.fhwa.dot.gov/saferjourney/library/countermeasures/23.htm.
Emergency vehicle codes require ambulances and fire trucks to slow down at intersections and remain alert for other users. For example, see:
Illinois General Assembly, “625 ILCS 5/ Illinois Vehicle Code, Section 11-205,” in Illinois Compiled Statues (Springfield).
In Pennsylvania, ambulances are specifically required to comply with stop signs and red lights.
Pennsylvania Department of Motor Vehicles, “Vehicle Code—Chapter 31,” (Harrisburg), 2.
Dan Burden and Paul Zykofsky. “Emergency Response: Traffic Calming and Traditional Neighborhood Streets,” (Sacramento: The Local Government Commission Center for Livable Communities, 2001)
Ryan Snyder et al. “Best Practices: Emergency Access in Healthy Streets,” (Los Angeles: Los Angeles County Department of Public Health, 2013). ↩︎ - Currently, many cities use SU-30 as the design vehicle on non-truck routes. For example, see:
“Design Criteria,” Seattle Right-of-Way Improvements Manual (Seattle: City of Seattle, 2012). ↩︎ - Low “crawl” speeds are referenced by Flexibility in Highway Design, (Washington, D.C.: U.S. Dept. of Transportation, Federal Highway Administration, 1997). ↩︎
- In New York City, approximately 10% of pedestrian injuries occur at “off-road” locations, such as on sidewalks or inside buildings.
Transportation Alternatives, “1,200 NYC Pedestrians Struck On Sidewalks Every Year,” accessed June 3, 2013, http://www.transalt.org/files/newsroom/streetbeat/askta/030425.html. ↩︎ - City Freight Master Plan (Portland: Portland Office of Transportation, 2006). ↩︎