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39 Minimum Delay (s): The minimum theoretical pedestrian type (i.e., a crossable gap and a yield) and a range of outcomes delay in seconds, defined as the time difference between (utilized and non-utilized). Data for a single crossing attempt when the trial started and when the first yield or crossable are often characterized by scarce data, where only certain gap was encountered by the pedestrian. events are represented. Once the data are combined for all Delay>Min (s): The delay beyond the first opportunity, participants, the analysis reports the average, minimum, defined as the time difference between first yield or crossable maximum, and standard deviation of performance at each gap encountered by the pedestrian and the actual crossing crossing location. initiation. The delay performance measures are measured in a tempo- ral dimension (in seconds). For those measures, aggregation The analysis further investigates two parameters that are is again to the level of the single participant at one crossing intended to describe the efficiency with which a crossing oppor- location. But in addition to the average, minimum, maxi- tunity is utilized for both gaps and yields: mum, and standard deviation, the analysis further reports the 85th percentile of the estimate. This is common practice for Latency (s): The latency is defined as the time between when the analysis of continuous variables in traffic engineering the last vehicle went through the crosswalk and the time the applications such as delay studies (Institute for Transporta- pedestrian initiated the crossing. tion Engineers 1994). Yield Lost Time (s): The yield lost time (YLT) is defined as the time between when a driver first yields and the time Performance Measure Example the crossing is initiated. Note that in some cases, pedestri- ans may prefer to cross only after a car has come to a full This section presents an illustrative example of the dif- stop (stopped yield), and so some inherent yield utilization ferent performance measures used in the analysis, which was time is expected. previously published in Schroeder and Rouphail (2010). The example in Figure 14 assumes a crossing attempt by a Finally, the analysis uses the rate of O&M interventions that single pedestrian who encounters 10 different vehicles at a represent a measure of pedestrian safety during the crossings. single-lane crossing. The study participants were at all times accompanied by a Figure 14 shows a time line of a pedestrian encountering certified O&M specialist who was directed to stop the partic- 10 hypothetical vehicle events. The time line proceeds from ipants if the crossing decision would have resulted in undue left to right, from the start of the trial until the last vehicle risk to pedestrian and/or driver. The resulting rate of O&M that interacted with the pedestrian crossed the plane of the interventions is defined as follows: crosswalk. Of the 10 vehicles, vehicles 2, 4, 7, and 8 yielded to the pedestrian, but none of these yields were utilized. Intervention rate (%): The number of times the O&M Vehicles 1, 3, 5, 6, 8, and 9 didn't yield even though a pedes- specialist intervened for a particular participant divided by trian was waiting at the crosswalk. No yield information is the total number of lanes crossed for a particular condi- available for vehicle 10 since the pedestrian had already walked tion. For example, one intervention over a set of eight lane across by the time it crossed the plane of the crosswalk. Con- crossings at the roundabout entry corresponds to an inter- sequently, the variable P(Yield) is calculated from 4 yields vention rate of 12.5%. divided by a total of 9 drivers that could have yielded and equals 44.4%. On the contrary, the variable P(Y_ENC) = 40% Most of the performance measures above are expressed as is calculated by dividing 4 yields by a total of 10 vehicles percentages, which could also be interpreted as a probability encountered in the trial. of a certain event taking place or a rate of occurrence of that The temporal separation between vehicles 23, 56, and event. For all percentage measures, the level of aggregation is 910 constitute 3 crossable gaps, the last of which was utilized on the level of the individual participant for all crossings by by the pedestrians. The gap from the start of the trial to vehi- that participant at a particular location. For example, a pedes- cle 1 and the gaps between vehicles 45 and 89 were below the trian who crosses the entry leg of a roundabout four times will crossable gap threshold. The measure P(CG) = 50.0% is calcu- have an average rate of yield encounters calculated from those lated by dividing 3 crossable gaps by 6 total gaps encountered. four crossing attempts. The same pedestrian will have a dif- P(CG_ENC) = 30.0% is calculated by dividing 3 crossable gaps ferent rate of yield encounters for the exit leg crossing and by a total of 10 events. also different entry and exit leg percentages for any additional The advantage of the P(Y_ENC) and P(CG_ENC) mea- approaches at the roundabout included in the study. The sures is that they have the same denominator (total number of aggregation to the leg per participant level is necessary to encounters) and are thus additive. This ensures a consistent ensure that the data point includes at least one of each event and objective definition of events. In the NCHRP Project 3-78A