Wide Outside Lanes

In urban areas, paved shoulders are not normally provided on major roads. A wider outside (or curbside) lane allows a motorist to safely pass a cyclist while remaining in the same lane and this can be a significant benefit and improvement for cyclists, especially more experienced riders. A wider outside lane also helps trucks, buses, and vehicles turning onto the major road from a driveway or wide street.

Critical dimensions

  • 14 feet (4.2m): recommended width for wide outside lane width must be useable and measurement should be from the edge line or joint of the gutter pan to the lane line
  • 15 feet: (4.5m) preferred where extra space required for maneuvering (e.g. on steep grades) or to keep clear of on-street parking or other obstacles.

Continuous stretches of lane wider than 15 feet may encourage the undesirable operation of two motor vehicles trying to squeeze into one lane. Where this much width is available, consideration should be given to striping bike lanes or shoulders.

Critical issues and Frequently Asked Questions

Is a wide outside lane safer/better than striping a bicycle lane?

The simple answer is that there have been too few studies to answer this question definitively, but the studies that have been done suggest that while there may be no direct affect on safety, both bicyclists AND motorists are more comfortable and confident on roads with striped bike lanes. In An Evaluation of Shared-use Facilities for Bicycles and Motor Vehicles, by David Harkey and Richard Stewart, the authors updated studies from the 1970s investigating the interaction of motorists and bicyclists in different roadway conditions and came to the following conclusions:

  • Motorists are less likely to encroach on the adjacent lane when they are passing a bicyclist on facilities with pave shoulders or bicycle lanes
  • Motorists have less variation in their lane placement when they are passing a bicyclist on a paved shoulder or bicycle lane facility
  • Bicyclists are more likely to ride further from the edge of the roadway in a bicycle lane or on a paved shoulder than they do in a wide curb lane (providing a greater margin of safety to avoid obstacles and making them more visible)

Two studies of "levels of service" for bicyclists also provide useful insight into answering this question.

In Real Time Human Perceptions: Towards a Bicycle Level of Service, by Landis, Vattikuti, and Brannick, the authors have developed a level of service model for bicyclists that incorporates cyclists' perceptions and feelings of comfort and safety. In studying the impact of striping a bike lane or a paved shoulder the authors fund that increasing the width of a travel lane to 16 feet increased the level of service to the bicyclist by 13 percent. By striping a four foot bike lane on the same roadway, the level of service increased by 31 percent.

Development of the Bicycle Compatibility Index: A Level of Service Concept, Final Report, by Harkey, Reinfurt, Knuiman, Stewart, and Sorton. The Bicycle Compatibility Index incorporates the geometric and operational variables considered by adult bicyclists to be important in terms of their comfort level when riding on the streets with motor vehicles. The authors note that the variable with the largest effect on the index is the presence or absence of a bicycle lane or paved shoulder that is at least 0.9m wide. Striping a bike lane reduced bicyclist stress levels by almost one unit on a five unit scale.

Additionally, marking a bike lane helps give motorists the signal that bicyclists are legitimate users of the roadway, and can help preserve a space for bicyclists on the road. One it is marked, it is harder to 'steal' that space from bicyclists for other uses. Of course, if a bike lane is marked, there is also more of a legal obligation for the agency to maintain the markings and surface condition at an acceptable level for bicycle riding.

Can I provide a wider outside lane by reducing the width of other travel or turning lanes?

The simple answer to this one is "yes." The AASHTO bicycle guide says "restriping to provide wide curb lanes may also be considered on some multi-lane facilities by making the remaining travel lanes and left turn lanes narrower." The guide does caution you, however, to do this only after careful consideration of traffic characteristics and other factors. So, for example, a relatively low volume road with two 12 foot travel lanes (in one direction) could be restriped with a 13/11 foot or 14/10 foot pattern without adding anything to the width of the roadway.

For example, Phoenix has used a 10 ft lane width for more than 30 years as standard lane width for interior lanes and for the two-way left turn lane (TWLTL). Lanes next to a raised median or a curb lane are at least 12 feet wide. The 10 ft lanes have not caused any adverse safety impact, and may even act to a small extent in a speed reducing capacity. One problem with this has been that the narrower lanes have been used to add an extra vehicle lane. In more recent years, there have been a few projects to use narrower (10 ft) lanes to provide a bike lane as a part of 'road diets'.


Check out the Wisconsin Bicycle Facility Design Handbook for a discussion of the use of wide outside lanes.