The Public Works Department conducts "School Counts" during each time period when children are going to and from school. The data gathered includes the number of children crossing and the total number of opposing cars and trucks. The data is summarized and a crosswalk rating is derived by using the following:
P = Sum of the pedestrians during the peak a.m. 1/2-hour period and the peak p.m. 1/2-hour period.
V = Sum of the cars during the peak a.m. 1/2-hour period and peak p.m. 1/2-hour period, plus three times the sum of trucks and buses during the same peak 1/2-hour periods.
S = Adjusts for the speed limit through the crossing. The base speed limit of 25 mph has a factor of 1.00. For each 1 mph increase in speed a factor of 0.05 is added to the base factor of 1.00. Thus, the factor used for a posted speed limit of 35 mph is 1.00 + (10 x 0.05) = 1.50.
W = Adjusts for width of street measured from external curb to external curb. The base street width of 20 feet has a factor of 1.00. For each 1 foot increase in width a factor of 0.05 is added to the base factor of 1.00. Thus, the factor for a street width of 26 feet is 1.00 + (6 x .05) = 1.30.
1,000 - In order to keep the resulting product of this formula from becoming unwieldy, it is divided by 1,000, a constant.
Example of a Crosswalk Evaluation
Peak 1/2 Hour Periods: 8:20 to 8:50 A and 2:30 to 3:00 P
Safe Gaps: AM Peak 1/2 Hour 54 PM Peak 1/2 Hour 57
Peak (AM + PM) 1/2 Hour Total Volumes: 11-P; 436-C; 9-T
Street Width = 30 feet. Factor = 1.50
Speed Limit = 35 mph. Factor = 1.50
Rating = 11 x 463 x 1.50 x 1.50 = 11.5
Note: Installation of a marked and signed crosswalk requires a minimum rating value of 10.
Installation of a pedestrian actuated signal requires a minimum rating value of 100.
Studies indicate that elementary school children walk at a rate of 3.5 to 5.3 feet per second. To establish the minimum safe gap time for school children, the street width is divided by the walking rate of 3.5 feet per second and then the pedestrian perception/reaction time of 3 seconds is added. The safe gap is computed using the following:
After the safe gap has been computed, a study is conducted during the periods when children are going to and from school. The study measures the actual length of gaps between the moving vehicles and the number of pedestrians accepting the various size gaps to cross. Those intervals between vehicles that are less than the computed safe gap are called "blockades". The sum of all blockades during the study period is the pedestrian crossing delay. The percent of delay is of value in cases where there are few, but very long, gaps. Figure 1 illustrates the Safe Gap-Blockade Concept.