2 Delay Time Delay time is the additional travel time experienced by a driver

2 delay time delay time is the additional travel time

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2. Delay Time : Delay time is the additional travel time experienced by a driver during congestion or slowed traffic resulting from the work zone. 3. Queue Length : A queue is a group of vehicles that are traveling very slowly or are stopped. In this study queuing conditions are defined for speeds less than 10 mph similar to Notbohm et al. ( 11 ). Maximum and average queue lengths in work zones are then computed based on the extent of the queue during different time periods. 4. Crash Frequency: The number of crashes per work zone duration is defined as the crash frequency. Alternatively, crash rate, which includes the traffic exposure, may also be used. 5. Speed : Measures such as mean speed, 85 th percentile speed, compliance rate and speed variability are common speed measures. Mean speed is the arithmetic mean of the vehicle speeds in a given time interval. The 85 th percentile speed can be computed using the cumulative speed plots and can be compared with the posted speed limit. Some agencies Congestion is frequent at the location Work zone results in high delays or crashes Alternative routes are available Demand is frequently at or above road capacity
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5 prefer the use of 85 th percentile speed than the average speed because speed limits are often based on the 85 th percentile speed. Speed compliance rate indicates the percentage of drivers traveling below the speed limit. The compliance rate is particularly useful when work zones have lower posted speed limits than normal. Speed variance gives the dispersion of speeds with respect to the mean speed. Speed variance has also been found to be related to safety. All the aforementioned speeds are correlated. It is up to the agency to decide which speed measure best captures the agency’s goals. In addition to these measures, other measures can be evaluated depending on the agency needs. For example, a few studies have used travel time as a surrogate for delay, emissions, emergency response time, changes in demand, and number of forced merges. Data Collection Traffic sensors that collect traffic flow, speed, and occupancy are key to the chosen performance measures. Additional equipment such as queue detection trailers may also be needed for accurately measuring queue lengths. In Table 2, a summary of the data needs for each performance measure and the type of ITS communication impacting them are shown. For example, diversion rates can be computed using traffic flow data collected from traffic sensors deployed on the mainline and ramps. ITS technologies such as dynamic message signs (DMS) can be used to communicate the availability of alternative route and the delay on the existing route influencing driver behavior. To evaluate the effectiveness of ITS in a work zone, there should be a control measurement without the use of ITS. State agencies are usually hesitant to turn off an ITS system after its deployment in the field. This is presumably due to the public expectation of receiving traveler information through ITS. However, turning off ITS equipment is essential in order to
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