Cost-Effective Performance Measures for Travel Time Delay, Variation, and Reliability (2008) / Chapter Skim
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Pages 41-50
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From page 41... ...
The road segment travel times section provides two methods for estimating segment travel times, a sketch planning method and a planning method. The sketch planning method requires only average daily traffic, signal spacing, and number of lanes.
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on road segment; Access Points = Average number of freeway interchanges Per Mile per mile for freeway segment. Treat partial interchanges as full interchanges for purpose of computing the average access points per mile; Signals Per Mile = Average number of traffic signals per mile on road segment; Arterial Class I = Signalized arterial street with at least 1 signal every 2 miles, and with posted speed limit in excess of 40 mph; Arterial Class II = Signalized arterial street with at least 1 signal every one-quarter mile, and with posted speed limits of 30 to 40 mph, inclusive; and Arterial Class III = Signalized arterial street with at least 1 signal every one-quarter mile, and with posted speed limit less than 30 mph.
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From page 43... ...
. The other equation is for facilities with lower posted speed limits.
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for multilane rural roads with 60 mph FFS; = 2,100 (pcphl) for multilane rural roads with 55 mph FFS; = 2,000 (pcphl)
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Option 2d. Capacity Equation for Signalized Arterials The following equation is used to compute the one direction capacity of any signalized road with signals spaced 2 miles or less apart: Capacity (vph)
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Step 3. Compute Average Speed If it is desired to compute mean speed for each hour of a day, then once the link capacity and free-flow speed are known, the updated BPR equation (Equation 6.16)
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The segment to segment transition delay also can be neglected for through travel along a signalized arterial street since the methods described for estimating segment speeds include a nominal delay per signal in their estimates. If the analyst is evaluating a route that involves left turns on signalized streets, the analyst may wish to improve the accuracy of the travel time estimate by adding a nominal delay per left turn to the estimated total travel time for the trip.
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From page 48... ...
So an approximation of the FFS would be the mean travel time and speed measured under low flow conditions. This method of measuring speed and travel time includes nominal delays at signals due to modest amounts of traffic on the main street and the side street.
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From page 49... ...
. The variance of the travel time is equal to the variance in the v/c ratio times the square of the slope of the linear traveltime function for the facility.
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From page 50... ...
6.24) 6.4.3 Computation of Reliability Metrics The following equations are for use with forecasted travel times, where only the mean and variance are known.
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