2010-04-14 CC Packet - Laserfiche WebLink

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August 1, 2007 TRAFFIC ENGINEERING MANUAL The cars checked should be the ones in which driN ers are choosing his /her own speed and which have at least 6 to 9 second headways. When a line of vehicles moving closely behind each other passes the speed check station, only the speed of the first -vehicle should be checked since the other drivers may not be selecting their own speeds. Cars involved in passing maneuvers should not be checked because they are probably traveling at an abnormal rate of speed. Speeds of trucks and buses may be recorded separately, and are not normally used in the calculations to determine the posted speed. Trucks and buses are usually a minor percentage of the traffic population and their speeds may not be representative afthe rest of the traft being Audi - The truck and tags data is useful, however- in the evaluations of the speed profile, especially if the volume of trucks and buses is significant. For both rural and urban areas, a map should be prepared showing the length of study area, north arrow scale.. in place speed Iiinit zones proposed speed limits,. and the speed }emits adjoinutg the Audy area- In urban or suburban areas, an accurate log should be made of all traffic signs, traffic signals, intersections, curves over 2 degrees, grades over 3 percent, and location of businesses and residences. Rural zones should have a log only if a speed limit, other than a statutory value, is being recommended. Normafly in. rural areas the Transvva.taiilvu. Information: System ( TJ.S._) log point listing is adequate to show intersection spacija& along with some general comments about land use adjacent to the roadway. All reports should contain physical cbaracteristics such as surface type and width, shoulder type and width eouchtion of both, number of lanes, location of turn lames, and ADT. The lotion, type and numerical limit of all advisory speed signs should be logged. On trunk highways, the crash rates should: be submitted far the study area. A comparison should be made with the - district annual average crash rate by highway type. Ifthe study area is significantly above the average rate_ an crash summary report should be printed out and analyzed. Average rates are not published for local roads, but summary printouts can be obtained. Anytime the request for an investigation is the result of crash experience, the summary printout should be included in the study. Adeauate consideration may then be given to corrective measures, the degree of enforcement emphasis needed and the impact of any speed changes. 13 -6.06 Conditions Justifying Variations from the 85th Percentile Speed The approved posted speed should normally be the nearest value below the 85th percentile speed which ends in 5 or 0. Conditions which justify varying the zone speed from the 85th percentile speed are: 1. If the 85th percentile speeds for adjacent speed check stations are approximately the same, they may be averaged to determine the zone speed. This method may also be used when differences occur in opposing directions. If directional speed differences are greater &n 10 mph; different zones may be established for opposing directions Enforcement officiaels should be notified of this decision and their reaction submitted with the investigation results. 2. Speed limits may be established 5 mph under the 851h percentile speed when there is a high crash record involving crashes of a type that would be reduced by enforcement of a lower speed limit. Crash severity rates should be calculated for each of the preceding 3 -5 years and demonstrate an increasing progression of severity. These rates and the computer printouts used to determine them should be submitted with the report. A lower speed limit should be established only after the commitment of reasonable enforcement is assured. At locations where traffic volumes are very light and a sufficient number of cars cannot be checked in the two hours that the speed check station is operated, the 85th percentile speed may not be reliable_ In these cases: test runs should be done by experienced traffic engineering personnel. Test run speeds should be selected in 5 mph increments. Ideally, two test runs at each speed should be conducted out in each direction by two different drivers. The vehicle should have a calibrated speedometer. Test runs should provide a basis from which to select a reasonable and safe speed. This data and the recommended speed with a short Justification statement should be marked on the report. 13-25 K:3