Bridge Width

Bridge width is the total width of all lanes and shoulders on the bridge, measured between the points on the bridge rail, curb, or other vertical elements that project the farthest onto the roadway (Figure 12). A bridge width that meets adopted criteria maintains the minimum acceptable lane and shoulder width for the particular design condition as defined by area, functional class, design speed, and traffic volume.  A design exception is required when a bridge is proposed to be constructed with narrower lanes, shoulders, or both.

Potential problems associated with narrow bridges are twofold. Relatively short bridges represent a discontinuity that may affect driver behavior. The narrowed cross section can make some drivers uncomfortable and cause them to dramatically reduce speed, increasing the risk of rear-end crashes and degrading operations on high-speed, high-volume facilities.  The bridge rail may be close enough to the travel lanes to cause drivers to shy towards the centerline or into adjacent lanes (Figure 13). The bridge infrastructure itself is closer to the edge of pavement and thus represents a roadside hazard. Even when properly designed and delineated, there is an increased risk of a roadside collision with a bridge end closer to the edge of traveled way.

A second set of concerns is evident for longer bridges (say, greater than 500 feet in length). The safety and operational concerns at narrow bridges are similar to those on roads with narrow shoulders. There may be inadequate space for storage of disabled vehicles, enforcement activities, emergency response, and maintenance work.  The lack of shoulder width on the bridge may make it impossible to avoid a crash or object on the roadway ahead.  In addition, options are limited for non-motorized users such as bicyclists, forcing them onto the traveled lanes or close to the bridge rail.

Narrow bridges on horizontal curves can have limited horizontal stopping sight distance past the bridge rail (Figure 10).  Operations can be degraded, particularly on long bridges on high-speed roadways, because of speed reductions as drivers enter the narrowed cross section as well as a decrease in driver comfort on the bridge.

Figure 12 is a photo of a long bridge with an arrow indicating bridge width as the total width of lanes and shoulders on the bridge up to the first vertical elements, in this case the curb, that project out into the roadway.

Figure 13 is a photo showing three cars on a rural two-lane highway, traveling in the same direction.  The middle car’s left front wheel is crossing the painted centerline into the opposite lane as the vehicle travels over a short bridge, as the driver shys away from the bridge rail.  The other two cars that are not on the bridge are centered within the lane.

Summary

Table 10 summarizes the potential adverse impacts to safety and operations of a design exception for bridge width.

Freeway:  high-speed, multi-lane divided highway with interchange access only (rural or urban).
Expressway:  high-speed, multi-lane divided arterial with interchange and at-grade access (rural or urban).
Rural 2-Lane:  high-speed, undivided rural highway (arterial, collector, or local).
Urban Arterial:  urban arterials with speeds 45 mi/h (70 km/h) or less.

Substantive Safety

In evaluating the potential substantive safety of narrow bridges, the designer should consider the two types of conditions described above. For short bridges, the safety risk can be modeled by use of the Roadside Safety Analysis Program (see the AASHTO Roadside Design Guide).  Based on traffic volumes and the widths in question, a designer can estimate the relative increased risk of the bridge end closer to the traveled way.

For longer bridges, the designer can reference information in the shoulder width section, such as Figure 11, to gain an understanding of the incremental increase in safety risk with a narrower dimension for the combination of lane and shoulder width.

Bridge Width Resources

• A Policy on Design Standards Interstate System, AASHTO, 2005.
  
  
• A Policy on Geometric Design of Highways and Streets, AASHTO, 2004.
  
  
• A Guide for Addressing Head-On Collisions, NCHRP Report 500, Volume 4, Transportation Research Board, 2003.
  
  
• Roadside Design Guide, AASHTO, 2002.
  
  
• Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.
  
  
• Highway Safety Design and Operations Guide, AASHTO, 1997.
  
  
• FHWA Roadside Hardware Web site http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/
  
  

Back to the Table of Contents