(A) Spot speed studies are to be analyzed to determine the eighty-fifth percentile speed and the upper limit of the ten mph pace. The EIGHTY-FIFTH PERCENTILE SPEED is defined as the speed at which 85% of the vehicles are traveling at or below. The TEN MPH PACE is defined as the ten mph range containing the most vehicles.
(B) Spot speed studies should be made as close to the center of the proposed zone as is practicable. If the zone is in excess of one mile in length, studies should be made every mile. If the difference in prevailing speed between the two adjacent locations is less than five mph, the lower value should be used. If the difference in prevailing speed is over five mph, consideration should be given to designating two separate zones.
(C) Care must be exercised to be sure that the data is collected in such a manner and at such times that it is a true indication of normal conditions. Such conditions are assumed to prevail under clear weather conditions, dry pavement, following the morning peak traffic hours and prior to the evening peak traffic hours on any weekday except holidays. The traffic study shall not be taken the day prior to a holiday or the day after a holiday. Observations should not be made immediately following a spectacular accident or during a period of greater than normal police enforcement. Every effort should be made to conceal the fact that speeds are being recorded.
(D) Speeds should be observed for a total of 100 passenger cars and pickup trucks in each direction (a maximum of three hours of observation can be imposed on low volume highways). Speed of trucks over four tons shall not included in the data for determining an altered limit.
(E) Where data collection is difficult on low traffic volume rural subdivision streets, the sample sizes reduced. Speed studies need not be conducted on every subdivision street each time a zone is proposed. Speed studies conducted on a representative sample of subdivision streets would be acceptable. Speed data or other criteria from these representative streets could then be applied to any other similar streets.
(Res. passed 3-13-2007)