Safer Roadways: A Complete Streets Guide 38 For design purposes, the WB‐40 (wheelbase 40 feet) is appropriate unless larger vehicles are more common. On bus routes and truck routes, designing for the bus (city bus or similar) or large truck (either the WB‐50 or WB‐62FL design vehicle) may be appropriate, but only at intersections where these vehicles make turns. For example, for intersection geometry design features such as corner radii, different design vehicles should be used for each intersection or even each corner, rather than a “one‐size‐fits‐all” approach, which results in larger radii than needed at most corners. Figure 2-9 illustrates the turn radii of a city bus. The design vehicle should be accommodated without encroaching into opposing traffic lanes. It is generally acceptable to have encroachment onto multiple same direction traffic lanes on the receiving roadway. Figure 2-10 illustrates a corner radius with acceptable encroachment onto multiple same direction traffic lanes, as well as the distance (in seconds) for select size wheelbase vehicles to turn the corner. Figure 2-11 shows a recessed stop line, which allows a city bus to encroach on opposite flow traffic lanes to maneuver around the given corner.
Figure 2-11 Recessed Stop Line (NATCO)
Figure 2-10 Corner Radii (NATCO)
It also may be inappropriate to design a facility by using a larger “control vehicle,” which uses the street infrequently, or infrequently makes turns at a specific location. An example of a control vehicle is a vehicle that makes no more than one delivery per day to a business. Depending on the frequency, by under designing for the control vehicle, the vehicle can be allowed to encroach on opposing traffic lanes or make multiple point turns. HORIZONTAL CLEARANCE/CLEAR ZONE Horizontal clearance is the lateral distance from a specified point on the roadway, such as the edge of the travel lane or face of the curb, to a roadside feature or object. The clear zone is the relatively flat unobstructed area that is to be provided for safe use by errant vehicles. In Suburban Town (C3T) and Urban General (C4) contexts, horizontal clearance based on clear zone requirements for rural and suburban highways is not practical because these areas are characterized by more bicyclists and pedestrians, lower speeds, more dense abutting development, closer spaced intersections and accesses to property, higher traffic volumes, and restricted rights‐of‐way. Therefore, streets with curbs and gutters in these areas do not have sufficiently wide roadsides to provide clear zones. Consequently, while there are specific horizontal clearance requirements for these streets, they are based on clearances for normal operation. Properly designing self-enforcing streets through Complete Streets will reduce, if not eliminate, errant vehicles. The minimum horizontal clearance is 1.5 feet measured from the face of the curb. This is primarily intended for signposts and
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