Question

Challenges encountered throughout process of collecting data, computing data, and using Highway Capacity Software (HCS)

Answer 1

The geometric, traffic and signal input data were entered into HCS 2000 software to estimate the predicted control delay to compare it to the field control delay based on the 15 minutes time interval and 216observations. Using the default saturation flow rate and bus PCE value, the relationship between the field and predicted delays is shown in Figure (1). It can be seen that, for low delay ranges (up to 20 seconds), HCS tends to over- estimate control delay. For higher delay ranges, it has a mix of under-estimations and over-estimations, with some points that are severely over-estimated.The measured field delays were regressed against the predicted ones producing the results shown in Table (4). The intercept is significantly different from zero and the slope is significantly different from 1.000 at 95%confidence. The R² value indicated that HCS explains only about 21.6% of the variability in control delay. The paired t-test was also used to see if the average deviations between filed and predicted control delay are significantly far from zero. the mean of differences is –10.29 sec.,with a P-value of 0.000, so the hypothesis that thedeviations are equal to zero is rejected at 95%confidence.The two calibrations (basic saturation flow rate and bus PCE factor) were also combined together to find their effect on the HCS delay estimation.

A total of 54 hours of data were collected from 18 approaches at 5 signalized intersections in Irbid city,Jordan. Intersections were selected based on the maps and traffic counts provided by Irbid Municipality. By looking at the plots of traffic volumes versus time at each intersection, it was possible to determine differnt levels of traffic volumes: low, moderate and high, in order to cover a wide range of data. Geometric characteristics were obtained through field measurements during off peak periods. Intersection geometry included number of lanes, lane width, island width, … etc. Data were collected during the sunny days in the spring of the year 2002 from locations with good pavement conditions.

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