2010-04-14 CC Packet - Laserfiche WebLink

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Aujust 1, 2007 TRAFFIC ENGINEERING MANUAL Speed: li mits less than the midpoint of the pace, should generally be avoided because they tend to increase the relative speed differentials among vehwles arid, therefore, the potentx`al for vehicle conflicts, Physical features may dictate the regulatory speed limit. Where a continuous curvilinear section of roadway with short connecting tangents exists, a regulatory speed limit should be posted to aid the motorist in determining an appropriate speed to travel. On a section of road with only in- frequent horizontal curves, "curve or "turn" warning signs with correct advisory speed plates should be erected, with speed zoning determined by the essentially tangent alignment of the road and other factors. Safe speeds on the curves should be determined using a ball -bank indicator. Other physical features which affect speed are roadway surface conditiorn, shoulder width, lane width and obstacles near the road. The effects of these factors, are not easily measured but are reflected by checking the prevailing speed. Traffic characteristics such as turning vehicles, traffic volumes, parking, traffic signals, and pedestrians, all affect speed. Intersections by themselves do not necessitate a r'1Z".k.A ; ,t . in speed. but the trafc characteristics, that occur near intersections do affect speed. It is difficult to measure the impact of any one of these factors or the collective affect. The varying speeds necessary for each reaction emphasizes why the study should be done in an off -peak period, so as to minimize this wide variation in speed.. Again, the 95th percentile speed will reflect the maxi ruin safe speed for the roadway without measuring the impact of each factor: Crash experience should be reviewed for the preceding 3 -year period. Contrary to popular belief, speed in itself is not a major cause of crashes. There is a consensus of professional opinions that many speed - related crashes result from both excessively low and high speeds_ Many requests for a speed study will be initiated by an emotionally aroused public after a specific crash_ It is important to review the crashes for the study area and determine ifthere is a problem that should be resolved rather than changing the speed limit. Typically, speed reductions may decrease the severity of crashes but not necessarily the frequency. 13 -6.05 lnvestigaftn Rrocedum When an engineering and traffic investigation is requested by local authorities, the exact length, as requested, must be investigated. If that length is too short to provide for a meaningful study, then the requesting authority should be notified and an amended resolution passed. On the Trunk Highway system, the area in question should: be studied and the entire control section should be test driven to determine and verify the impact of any recommended speed change. The locationn of the speed check is singularly important because it determines whether or not a complete picture of the speeds in the area is being obtained. The speed check locations must be strategically located to show all the important changes in prevailing speeds. In urban districts and on approaches to cities they should generally be located at intervals not to exceed 1/2 mile, depending upon the locality and the uniformity of physical and traffic conditions. Trial runs through the area may be of help in determining the speed check locations. Care should be taken to select locations sufficiently removed from any stop signs, traffic signals, or other traffic flow interruptions that significantly affect operating speeds. Mid -block locations generally represent typical flow conditions for accurate sampling. In rural areas, the spacing of speed check locations may be at much greater intervals so long as they reflect the general speed pattern. One speed check located at each end and the middle point of the selected zone should be sufficient. Speed checks. on new or reconstructed highway sections should not be performed until it is apparent that the traf&' speeds have stabilized. Where speed limits are immediately necessary for-the correct operation ofthe new road wa_ . then design speed and test run data will be acceptable for a temporary speed limit authorization. Local authorities should be made aware of this procedure when the initial study is done. A minimum of 100 cars should be checked. On highways carrying low traffic volumes, (up to 1000 ADT) the minimum of 30 samples may be used and, in any event, the study should be discontinued after two hours. 132, 37