U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-10-041
Date: December 2010
Evaluation of Shared Lane Markings
CHAPTER 7. CONCLUSIONS AND DISCUSSION
The use of shared lane markings continues to grow, and it is apparent that communities are looking to expand ways in which the markings can be used. The purpose of this study was to evaluate the impact of several uses of sharrows on operational and safety measures for bicyclists and motorists.
Sharrows were evaluated in three cities and with three different intentions. In Cambridge, MA, the markings were used 10 ft from the curb and next to parked motor vehicles to determine if the spacing for bicyclists next to parked motor vehicles and motor vehicles in the traffic stream could be increased. In Chapel Hill, NC, the markings were used along a corridor with wide outside lanes to determine if the spacing for bicyclists from the curb and from hazardous drainage grates could be improved as well as if motor vehicle spacing from the curb could be increased. In Seattle, WA, the markings were placed in the center of the street at a downhill portion to determine if bicyclists could be encouraged to take control of the lane. In addition, the operational interactions between bicycles and motor vehicles were studied.
The various hypotheses that were explored included the following:
In Cambridge, MA, there appeared to be safety effects associated with the installation of the sharrows 10 ft from the curb on Massachusetts Avenue, which is a busy street with frequent parking turnover. Some 94 percent of bicyclists rode over the sharrows. Operating space for bicyclists appeared to increase when motor vehicles were shifted 14 inches further away from parked motor vehicles in the absence of bicycles after placement of the markings. The distance from bicycles to parked motor vehicles in the presence of a following motor vehicle increased 4 inches after placement of the markings for the inbound direction but very little for the outbound direction. The percentage of bicycles riding within 40 inches of parked motor vehicles in the presence of a following motor vehicle also decreased, but the effect was again seen only in the inbound direction. It was unclear why there was an effect in one direction but not the other. There was little difference in the mean spacing distance when motor vehicles passed bicycles in the after period; however, the percentage of motor vehicles within 40 inches of bicycles decreased.
Examining a number of variables related to the operational interaction of bicycles and motor vehicles appeared to reflect more segregated flow with less lateral movement of bicycles and motor vehicles in the after period. The sharrows appeared to show no effect on free-flowing motor vehicle speeds or on the distance that motor vehicles were parked from the curb when front and rear distances were combined. However, in the outbound direction, the distance between the back tire and the curb increased by an average of almost 2.3 inches, and this increase was statistically significant (p = 0.038). This finding may be related to the lesser effect of the sharrows for the outbound direction.
In Chapel Hill, NC, there were a variety of safety effects found after placement of the sharrows on a busy five-lane corridor with wide outside lanes and no parking. Similar to the results found in Cambridge, MA, the spacing of motor vehicles from the curb in the absence of bicycles increased about 7 inches in this wide-lane situation. The percentage of motor vehicles within 50 and 60 inches of the curb was also reduced. The combined effect should give bicyclists more operating space. The 3 to 4 percent grade on the street had an effect on various outcomes. The percentage of cyclists riding over the sharrows was 97 percent in the uphill direction and 88 percent in the downhill direction. There was a tendency for cyclists to ride closer to the curb uphill, which was possibly due to a large speed differential between the bicycles and passing motor vehicles. There was more of a tendency for bicyclists to ride further from the curb going downhill, but the overall change in mean spacing (both directions combined) was 2.5 inches closer to the curb in the after period. It is worth noting that the mean distance of bicycles from the curb in the downhill direction in the after period was almost identical to the distance of the center of the sharrow from the curb. Related to the above findings, motor vehicles decreased their passing distance to bicycles in the after period. This appeared to lead to a smoother flowing traffic stream as motorists felt comfortable in passing bicyclists within the lane. It should also reflect less motor vehicle-to-motor vehicle interactions as fewer motor vehicles move partway into adjacent lanes when overtaking bicyclists in the after period. Conversely, this may decrease the comfort level of bicyclists operating on the street.
Many bicycle/motor vehicle crashes on this corridor occur as bicyclists riding on the sidewalk cross driveways and intersections. After placement of the sharrows, the percentage of sidewalk riding decreased significantly, but most of the effect was for the downhill direction.
In Seattle, WA, sharrows were placed in the center of the lane on a downhill portion of a busy bicycle commuting street. The experiment had three stages, and the second involved the addition of a 5-ft bicycle lane to the uphill portion of the street in conjunction with shifting the center line of the street. The overall effect was to narrow the travel lanes. The sharrows were added in the third stage, and the effects were somewhat surprising. Placement of the sharrows in the center of the lane on the steep downhill portion of the street was intended to encourage bicyclists to ride in the center of the lane as well as to increase their space from parked motor vehicles. However, only 15 percent of the bicyclists rode over the sharrow, although many tracked near the sharrow. The percentage of bicyclists riding in the center of the lane increased from 27 percent in the before period to 51 percent in after period 1 after the effective lane width had been narrowed by about 2.5 ft in the shift of the centerline to accommodate the addition of the uphill bicycle lane. The percentage of bicyclists riding in the center of the lane after sharrow installation reverted to 25 percent.
Additionally, there was little change in the spacing between bicycles and parked motor vehicles across all three periods. The percentage of bicyclists riding within 30 inches of parked motor vehicles increased from about 6 percent in the before period to about 12 percent for both after periods. Conversely, the percentage of bicyclists actually riding within the door zone was small for all three phases, and the average spacing between bicycles and parked vehicles was about 15 inches outside of the door zone. This may be a reflection of experienced bicyclists realizing that they could control the lane without actually being in the middle of the lane.
In this experiment, the narrowing of the lane in the downhill direction in after period 1 may have had more of an effect on the interactions of bicyclists and motorists than the subsequent sharrow placement. A number of the variables associated with the interactions showed positive indications (e.g., less direction changes and yielding by both bicyclists and motorists) so that the overall flow on the street may be somewhat smoother.
Seattle, WA, is known as being a bicycle friendly city, and having experienced motorists who are used to driving with bicyclists in the travel lanes may play a role in these results. In the before period, experienced bicyclists were already staying out of the door zone, and their mean spacing from parked vehicles varied only slightly after sharrows were installed.
Sharrows can be used in a variety of situations, and it is assumed that increased use should increase motorist awareness of both bicycles in the traffic stream and the possibility of bicycles entering 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 Gainesville, FL, as shown in a previous study.(4) Safety effects were apparent in variables related to the interactions between bicycles and motor vehicles. 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 of utmost importance to evaluate and report those experiments so that more data can be examined to provide improved guidance to users.