Characteristics of Emerging Road and Trail Users and Their Safety

Acceleration

The AASHTO Guide for the Development of Bicycle Facilities (p. 65) uses a bicycle acceleration rate of 0.5 to 1 m/sec² (1.5 to 3.0 ft/sec²) in its equation to determine the minimum green time.⁽²⁾ Table 10 below shows the observed 85th percentile acceleration rates by user type and distance, in 6.1-m (20-ft) increments. Bicyclists met or exceeded the AASHTO range for distances of up to 12.2 m (40 ft). At greater distances, the acceleration rates were much lower and fell short of the value range. This is an important finding with its relevance explained in the "Discussion" section.

For all distances, recumbent bicyclists had the highest 85^th percentile acceleration rates and manual wheelchairs, as expected, had the lowest 85^th percentile acceleration rates. For the initial distance traveled, 0 to 12.2 m (0 to 40 ft), hand cycles, manual wheelchairs, and Segways had acceleration rates slower than the lower end of the range used by AASHTO (i.e., slower than 0.5 m/sec2 (1.5 ft/sec²)).

^2. American Association of State Highway and Transportation Officials. Guide for the Development of Bicycle Facilities. American Association of State Highway and Transportation Officials, Washington, DC, 1999.

Table 10. 85^th percentile acceleration rates (m/sec²).

NOTE: This table includes only active participants.

Table 11 shows the time required for various path users to cover a given distance. This affects calculations for both minimum green time for traffic signals and for pedestrian clearance intervals. The AASHTO minimum green time calculation was performed using the AASHTO 0.5-m/sec2 (1.5-ft/sec²) acceleration rate. The values in the "Pedestrian Clearance" row were calculated using a walking speed of a constant 1.2 m/sec (4 ft/sec). Again, recumbent bicyclists had the lowest 85^th percentile elapsed times and manual wheelchair users had the highest 85^th percentile elapsed times. At signalized crossings, pedestrian signals are needed to accommodate the slower travel speeds of manual wheelchair users and pedestrians. For users likely to be operating on the roadway, the critical users for minimum green time would be hand cyclists, as they had the highest elapsed time of the vehicular-type devices. For pedestrian clearance intervals, manual wheelchair users would be considered the critical users; compared to other users, manual wheelchair users take the longest time to cover any given distance.

Table 11. 85^th percentile elapsed time (sec).

NOTE: This table includes only active participants.