U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-11-039
Date: April 2011
Evaluation of Pedestrian and Bicycle Engineering Countermeasures: Rectangular Rapid-Flashing Beacons, HAWKs, Sharrows, Crosswalk Markings, and the Development of an Evaluation Methods Report
CHAPTER 5. SHARED LANE MARKINGS
This chapter summarizes the FHWA report, Evaluation of Shared Lane Markings, FHWA-HRT-10-041.(22)
Shared lane markings, widely referred to as sharrows, are intended to convey to motorists and bicyclists that they must share the roads on which they operate. The markings create improved conditions by clarifying where bicyclists are expected to ride and by notifying motorists to expect bicyclists on the road. Figure 6 illustrates a generic sharrow as it appears in the 2009 MUTCD.(2) This study evaluated the impact of several uses of shared lane markings, specifically the sharrow design, on operational and safety measures for bicyclists and motorists. Experiments were conducted in Cambridge, MA, Chapel Hill, NC, and Seattle, WA.
Figure 6. Illustration. Generic version of a sharrow.
In 2008, NCUTCD recommended the inclusion of shared lane markings in the next version of MUTCD.(23) The recommendation was made with limited research conducted only on an 11-ft spacing from the center of the shared lane marking to the curb to prevent a bicyclist from striking an opening door of a parked motor vehicle (i.e., a dooring crash).(24) The 2009 MUTCD includes a provision for shared lane markings, specifically the sharrow design, with guidance that the markings should be placed at least 11 ft from the curb face or the edge of the pavement on a street with parallel parking.(2) On streets with no parking and an outside lane less than 14 ft wide, the centers of the sharrows should be placed at least 4 ft from the curb or edge of the pavement.
Many cities and States have started implementing shared lane markings to encourage the safe coexistence of bicyclists and motorists. However, few localities have formally evaluated the impact of these markings on safety or operations. The following hypotheses were explored for sharrows:
Separate evaluations were conducted in three U.S. cities. In Cambridge, MA, there was interest in experimenting with the placement of sharrows 10 ft from the curb to prevent dooring from parked motor vehicles. In Chapel Hill, NC, sharrows were placed on a busy five-lane corridor with wide outside lanes and no street parking. In Seattle, WA, sharrows were placed in the center of the lane on a downhill portion of a busy bicycle commuting street. Prior to the sharrows, a 5-ft bicycle lane was added to the uphill portion of the street, and the center line of the street was shifted.
The experimental design was to collect data on bicycles and motor vehicles operating in the traffic stream before and after sharrow installation. While it would have been desirable to have used an experimental design with comparison data, no adequate comparison sites were available. This is often the case in bicycle safety studies where slight differences in traffic flow, grade, pavement surface, or some other variable can greatly influence outcomes. One way to possibly obtain a comparison site is to install a treatment on part of a route and to use the remainder as a comparison. However, when a community is installing a treatment, almost invariably, the intention is to install the treatment along the entire route.
Local staff collected videotape data before and after sharrow placement. The bicycle was the basic unit of analysis. A number of measures of effectiveness and other attributes were examined. Videotape coding was performed to obtain information about the bicyclist and to examine the operations of bicycles and motor vehicles when a motorist was following or passing a bicyclist. In Cambridge, MA, and Seattle, WA, events related to the presence of parked motor vehicles were also examined, such as existing open doors and near dooring events, as well as motorists pulling into or out of parking spaces.
The following spacing data were also obtained from images extracted from the videotapes:
Chi-square tests were used to examine the distributions of variables before and after sharrow placement. Analysis of variance models were used to study the effect of sharrows on spacing and other performance measures. In these models, the independent variables included site characteristics, type of treatment, and a dummy variable indicating whether it was a before or after condition. The sign and significance of the coefficient of this dummy variable were used to assess the effectiveness of the markings. None of the data were combined across sites because of differences in the uses of the sharrows in each city.
Cambridge, MA, has many street cross sections where the recommended 11-ft spacing from the curb would not be feasible. Sharrows were placed 10 ft from the curb for about 2,500 ft on Massachusetts Avenue, which is a 4-lane divided street with approximately 29,000 vehicles per day, parallel parking on both sides, and a speed limit of 30 mi/h. Figure 7 shows Massachusetts Avenue before sharrows were placed on the street. Figure 8 shows a sketch of the before and after conditions. The intent was to determine whether the sharrows would improve spacing of bicycles and motor vehicles while also helping to prevent dooring.
Figure 7. Photo. Massachusetts Avenue condition in the before period.
Figure 8. Illustration. Cross section view of Massachusetts Avenue before and after sharrow installation.
A total of 94 percent of the bicyclists rode over the sharrows. Further results pertaining to the interaction of bicycles and motor vehicles included the following changes from before to after. All of the differences were from before-after distributions, and p-values are shown to denote statistical significance. The after period results were as follows:
Results pertaining to the spacing of bicycles and motor vehicles in the presence of a following motor vehicle in the after period included the following:
Results pertaining to the spacing of bicycles and motor vehicles in the absence of a following motor vehicle in the after period included the following:
Overall results from Cambridge, MA, indicated the following:
Sharrows were placed 43.5 inches from the curb along Martin Luther King, Jr. Boulevard (MLK) for 1.25 miles. MLK has a 5-lane cross section (4 travel lanes and a center 2-way left turn lane) with no parking, 27,000 vehicles per day, a speed limit of 35 mi/h, and periodic sunken drain grates next to the curb. There was a 3 to 4 percent grade where the videotape data were collected. The street had previously been resurfaced, and the outside lanes were marked nominally as 15-ft-wide lanes. The spacing of bicycles and motor vehicles from the curb and in situations where motorists passed bicyclists was of primary interest. Figure 9 shows MLK in the before period, and figure 10 provides a sketch of the before and after conditions.
Figure 9. Photo. MLK in the before period.
Figure 10. Illustration. Cross section of MLK before and after sharrow installation.
A total of 91 percent of the bicyclists rode over the sharrows—97 percent in the downhill direction and 88 percent in the uphill direction. Bicyclists riding uphill traveled slower and tended to ride closer to the curb. Further results pertaining to the interaction of bicycles and motor vehicles included changes from the before to the after period. All of the differences were from before-after distributions, and p-values are shown to denote statistical significance. The after period results were as follows:
Results pertaining to the spacing of bicycles and motor vehicles included the following:
Overall results from Chapel Hill, NC, indicated the following:
Sharrows were placed in the center of the lane 12.25 ft from the curb on a downhill section of Fremont Street, which is a 2-lane street that has a speed limit of 30 mi/h, 10,000 vehicles per day, a 3.6 percent grade, and parking on both sides of the street. The placement was meant to encourage bicyclists to take the lane while traveling downhill. Data were collected in two additional after periods following the before period. The centerline of the street was repositioned to allow a 5-ft bicycle lane and parking line to be installed on the uphill section of the street (after period 1). Sharrows were then added in the downhill direction (after period 2) since there was not enough width for bicycle lanes on both sides of the streets. Figure 11 shows a section of Fremont Street in the before period, and figure 12 provides a sketch of the before and after conditions.
Figure 11. Photo. Fremont Street in the before period.
Figure 12. Illustration. Cross section view of Fremont Street before and after sharrow installation.
A total of 15 percent of the bicyclists rode over the sharrow during after period 2. Further results pertaining to the interaction of bicycles and motor vehicles included the following changes from before-after distributions, with p-values denoting statistical significance:
Results pertaining to the spacing of bicycles and motor vehicles included the following:
Overall results from Seattle, WA, indicated the following:
Sharrows can be used in a variety of situations, and it is assumed that increased use should enhance motorist awareness of bicyclists, or the possibility of bicyclists, in the traffic stream. Results indicate that sharrows increased operating space for bicyclists. Sharrows have reduced sidewalk riding not only in the current study but also in a previous study in Gainesville, FL.(25) As communities continue to experiment with various uses of sharrows, it is recommended that researchers continue to create similar trials in other locations and traffic settings. Additionally, it is important to evaluate and report those experiments so that more data can be examined to provide improved guidance to users.