The effect of a ‘Share the Road’ sign on drivers passing bikes

This study looked at the effect of a “Share the Road” sign on the space that motorists gave to bicycles while overtaking.

There was a particular focus on roads fitted with centerline rumble strips which may cause motorists to pass closer to bikes. Cars were observed passing bikes on Michigan Highway and on a two-lane rural highway in Lower Peninsula. For each road two sections were chosen, one with a centerline rumble strip and one without. On each road section studies were carried out before and after a sign reading, “Share the Road” was fitted. Observations were made on clear days with good visibility.

Although the objective was to determine the effect of the “Share the Road” sign on drivers’ passing distance, other possible factors considered were:

  • The presence of rumble strips, hence there inclusion in the experimental design
  • Lateral position of bicycle in road, this was controlled by three controlled positions were used
  • Number of cyclists involved in passing maneuver
  • Type of vehicle (larger vehicles need more space and therefore tend to give bikes less room)
  • The presence of opposing traffic

Four pole-mounted, high-definition video cameras were used to monitor the passing distance of the cars. The distances were measured by project staff with results compared to ensure consistency. This method had a resolution of 0.15m. 1200 passing events were observed before the sign was installed and 1225 events were observed after it was installed.

Results are given with respect to:

  • Buffer distance: The lateral distance between the motor vehicle and the bike
  • Crowding: Defined as a buffer distance of less than 1.5m
  • Rightmost lane position: How far into the opposite lane the car moved
  • Speed: The speed of the massing motor vehicle

It was found that the sign did not significantly affect the buffer distance. The cyclist’s position did have an effect with the probability of crowding increased by 8 percent when riding at the outside edge of the hard shoulder and by 22 percent when on travel lane. The presence of opposing traffic increased the probability of crowding by 13 percent.


The interaction of motorists and bicyclists, particularly during passing maneuvers, is an area of concern to the bicycle safety community as there is a general perception that motor vehicle drivers may not share the road effectively with bicyclists. This is a particular concern on road sections with centerline rumble strips where motorists are prone to crowd bicyclists during passing events. One potential countermeasure to address this concern is the use of a bicycle warning sign with a “Share the Road” plaque. This paper presents the results of a controlled field evaluation of this sign treatment, which involved an examination of driver behavior while overtaking bicyclists. A series of field studies were conducted concurrently on two segments of a high-speed, rural two-lane highway. These segments were similar in terms of roadway geometry, traffic volumes, and other relevant factors, except that one of the segments included centerline rumble strips while the other did not. A before-and-after study design was utilized to examine changes in motor vehicle lateral placement and speed at the time of the passing event as they relate to the presence of centerline rumble strips and the sign treatment. Centerline rumble strips generally shifted vehicles closer to the bicyclists during passing maneuvers, though the magnitude of this effect was marginal. The sign treatment was found to shift motor vehicles away from the rightmost lane positions, though the signs did not significantly affect the mean buffer distance between the bicyclists and passing motorists or the propensity of crowding events during passing. The sign treatment also resulted in a 2.5 miles/h (4.0 km/h) reduction in vehicle speeds. Vehicle type, bicyclist position, and the presence of opposing traffic were also found to affect lateral placement and speed selection during passing maneuvers.


Driver behavior during bicycle passing maneuvers in response to a Share the Road sign treatment
Kay, Jonathan J. (Wayne State University, Department of Civil and Environmental Engineering, 5050 Anthony Wayne Drive, Detroit, MI 48202, United States); Savolainen, Peter T.; Gates, Timothy J.; Datta, Tapan K. Source: Accident Analysis and Prevention, v 70, p 92-99, September 2014

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