Calculating 85th Percentile Speed

Use the following procedure to calculate the 85th percentile speed:
  1. Add the tally marks as shown in the “Cumulative Total” column in Figure 3-2. Note that the marks are added from the bottom up.
  2. For each “Cumulative Total” column, calculate 85 percent of the total number of vehicles checked.
    Example: Figure 3-2 shows that 125 cars were counted in the northbound direction. So 85 percent would be 106 (125 x 0.85 = 106). Thus, the 106th car (counting up from the bottom) represents the 85th percentile speed.
  3. Determine the speed at which the car representing the 85th percentile was traveling. Again from the northbound example in Figure 3-2, the 106th car was traveling at 48 miles per hour. Thus, 48 miles per hour is the 85th percentile speed.
On the back of the Radar Motor Vehicle Speed Field Tally Sheet there is an “85th Percentile Calculation Table,” which may be used to readily determine the 85th percentile car if the total number is between 80 and 239.
In no case shall the 85th percentile speed be interpolated between two speeds in the M.P.H. column.
After the locations of the speed check stations have been decided upon and the speed checks have been made, the 85th percentile speeds should be calculated immediately in the field. By doing so, it is possible to get an idea of what the speed curve will look like and to determine if more speed check stations are needed.