Operational and Safety Impacts of Restriping Inside Lanes of Urban Multilane Curbed Roadways to 11 Feet or Less to Create Wider Outside Curb Lanes for Bicyclists (September 2011, Florida Department of Transportation)

Thought folks might be interested in the general findings on the amount of passing clearance motorists provided cyclists when there was a wider outside lane (closest to curb) and a narrower inside lane(s). Note that the roads investigated were high speed and multi-lane, and data was collected during peak hours.

The entire report can be downloaded here:
http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_RD/FDOT_BDK82%20977-01_rpt.pdf

Background: The operational analysis involved investigation of the influence of several site characteristics on the operational behavior of motorists when passing bicyclists on asymmetric curb-and-gutter roadways. The research team collected data on curb-and-gutter asymmetric four- to six-lane roadways with posted speeds ranging from 30 to 45 miles per hour. Conclusion: The results from descriptive statistics, 95% confidence intervals, and regression modeling, all point out that lateral separation between motor vehicles and bicyclists is highly influenced by the width of the outside through lane.

Lateral Clearance Findings

• The greatest lateral separation (averaging 5.5 ft) was obtained when bicyclists rode between three and four feet from the face of curb.
• An increase in the width of the outside through lane resulted in greater lateral separation between motor vehicles and bicyclists.
• As the volume of motor vehicles increased, lateral separation decreased.
• Motorists provided less separation to bicyclists when other vehicles were present in the inside lane.
• Motorists provided 0.5 ft additional lateral separation to female bicyclists and 0.35 ft additional separation to casually dressed compared to athletically dressed cyclists.

Lateral Shift Findings

• The amount of the motor vehicle body partially shifting into the inside lane was reduced with the increase in the width of the outside through lane.
• Passenger cars were observed to have the lowest amount of lateral shift when passing a bicyclist.
• Large trucks were observed to provide the greatest amount of lateral shift when passing, often slowing down and completely moving to the inside lane to allow sufficient lateral separation to bicyclists.
• Less lateral shift was observed with increased vehicular traffic volume.
• The tendency was for drivers to move left if they had the opportunity.

Motor Vehicle Lane Usage Findings

• Given acceptable gaps, there was a tendency of motorists to move from the outside through lane to the inside lane after recognizing that there was a bicyclist downstream.
• In the absence of a bicyclist, more vehicles (56.2% for 4-lane segments, 30.6% for 6-lane segments)) were observed to use the outside through lane.
• In the presence of a bicyclist, a considerable proportion of motor vehicles shifted to the inside lane before passing the bicyclist to avoid sharing the outside through lane with a bicyclist.
• Only 40.2% of vehicles used the outside through lane when a bicyclist was present for 4-lane segments; 25.7% for 6-lane segments.

Motor vehicle Speed Finding

• On average, drivers reduced their speeds (from 34.13 to 32.76 mph) when passing bicyclists to ensure safe passing maneuver and accelerated (from 32.76 to 36.86) after passing bicyclists.