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Evaluation of Shared Lane Markings

CHAPTER 2. LITERATURE REVIEW

Many cities and States have started implementing shared lane markings to encourage the safe coexistence of bicycles and motor vehicles. However, few localities have formally evaluated the impact of these markings on safety or operations. The San Francisco Department of Parking and Traffic conducted an evaluation of the following two shared lane marking designs on streets with adjacent parallel parking: (1) a bike-in-house design and (2) a bike-and-chevron design.⁽³⁾ First, researchers conducted assessments to determine appropriate spacing for bicyclists to be able to avoid the door zone, which is the area where bicyclists risk colliding with an open door of a parked vehicle. It was determined that the 85th percentile for the door zone extended 9.5 ft from the curb in the study areas (7 ft from curb edge to outside of vehicle and 2.5 ft occupied by an opened door). Thus, bicyclists needed to ride at least 30 inches from parked vehicles to be relatively safe from an opened door. The marking treatments were subsequently implemented with the center of the markings 11 ft from the curb face to suggest a bicycle tracking position. This distance was intended to accommodate the 85th percentile distance of door clearance (9.5 ft) plus 0.5 ft of shy distance (distance between the bicycle and nearest point of a motor vehicle beyond which the motor vehicle is not deemed to be an immediate hazard) and half of the average bicycle width (2 ft). The San Francisco, CA, study was a before-after evaluation where data were collected on six street segments before and after markings were introduced. Curb lane widths, including parking, ranged from about 17 to 19 ft on 4 four-lane roads, and the curb lane widths were 22 ft on 2 two-lane roads. Each of the streets had moderate (2,000–4,000 vehicles per lane per day) to heavy (more than 4,000 vehicles per lane per day) traffic. In each of these locations, the bike-in-house marking was painted along one side of the road, and the bike-and-chevron marking was painted on the other side. Both shared lane markings led to the following results:

• 25 to 35 percent fewer sidewalk riders.
• 3 to 4 inches more space between bicycles and parked vehicles.
• More than 2 ft of additional space between bicycles and passing motor vehicles in travel lanes.
• About 1 ft of additional space between motor vehicles in travel lanes and parked vehicles (no bicycles present).

There were also reductions in the proportions of wrong-way riders associated with the bike-and- chevron (similar to sharrow) design. Some potential confounding treatments were removed prior to the installation of the shared lane markings at two of the sites, but it is unknown whether these would have had an effect on before or after measurements and results. Because the bike-and-chevron marking was more readily understood by bicyclists to indicate a preferred travel path (although these conclusions were somewhat tenuous), this marking was the preferred choice and ultimately approved for inclusion in the California Manual on Traffic Control Devices.⁽⁵⁾ Other agencies have since adapted the bike-and-chevron design with some minor modifications, although some are still employing the bike-in-house design.

Sidewalk and wrong-way bicycle riding have been overrepresented in collisions with motorists turning right on red, driving out at a midblock location, and proceeding through a junction.^(6,7)

Hunter, Stewart, and Stutts found that both wrong-way riding and sidewalk riding were more prevalent at wide curb lane sites than at bicycle lane sites in a cross sectional study of operational factors and conflicts on the two types of facilities.⁽⁸⁾ While the study was not conclusive regarding higher incidence of wrong-way riding at wide curb lane sites in general, markings that would reduce the incidence of wrong-way riding and sidewalk riding at wide curb lane sites could nevertheless enhance the safety of these facilities.

In the late 1990s, Pein, Hunter, and Stewart conducted a before-after study of a variant of the bike-in-house marking implemented on a four-lane high-volume (35,000 vehicles per day) arterial street with a 30-mi/h speed limit in Gainesville, FL.⁽⁴⁾ The roadway had wide outside lanes 15 ft to the curb and no on-street parking. As a result, the lanes were wide enough to be shared side-by-side by bicycles and motor vehicles. However, a gutter pan had been paved over, and the 2-ft gutter pan area was included as part of the width of the outside lane. A seam 2 ft from the curb was present and conspicuous after the repaving, which influenced the decision of where to place the stencil. The center of the bike-in-house marking was placed 3.5 ft from the curb face, resulting in a 1.5-ft spacing from the old gutter pan seam. The marking was evaluated to determine whether it reinforced the correct direction of travel (with traffic) and reduced sidewalk riding by highlighting recognition of the wide outside lane as a bicycle facility. The other measures evaluated were the bicycle position from the curb, the space between overtaking motor vehicles and bicycles, and motor vehicle distance to the curb when no bicycles were present. Spacing distance of bicycles from the curb was measured with and without motor vehicles present in both periods. Potentially influential motor vehicles were present 82 percent of the time in the before period and 83 percent of the time in the after period.

There was a statistically significant increase in the proportion of bicyclists riding in the street in the correct direction (with traffic) from the before period (39 percent) to the after period (45 percent). Bicyclists riding in the street rode an average of 1.6 ft from the curb in the before period and 1.8 ft from the curb in the after period—a shift of about 3 inches. This change was statistically significant but not thought to be practically significant. However, there was also a larger proportion of bicyclists riding 1.75 to 2.5 ft from the curb, indicating that more bicyclists consequently had additional maneuvering space toward the curb in the event that motorists encroached into their space. This also potentially increased the comfort of bicyclists using the shared lane. Motorists allowed an average of approximately 1.5 inches additional space when passing bicyclists in the after period (6.1 ft) compared to the before period (6.0 ft); however, this difference was also not considered practically significant. The mean and median motor vehicle distance to the curb also increased slightly. Estimates of conflicts, adjacent lane encroachments, or motor vehicles completely changing lanes in order to pass were not studied in this evaluation.

Similar operational and spacing measures have been used in studies evaluating operational effects of bicycle lanes and wide curb lanes (without shared lane markings). It has generally been found both in comparative studies and before-after studies that the presence of a bicycle lane stripe reduces motor vehicle encroachment and increases tracking consistency for a given roadway width.^(9–11) The studies also report small bicyclist shifts away from the roadway edge or parked vehicles.^(11,12) The van Houten and Seiderman study examined the effects of sequential bicycle lane markings compared with a baseline of only a roadway center line and also found that there was less variability in bicycle tracking once the first bicycle lane stripe (toward the center line) was added.⁽¹²⁾ It is possible that the use of shared lane markings would have effects on motorist tracking and encroachment as well as on bicyclist position and riding direction.