Data collection

Two types of data are generally collected at unsignalized intersections, namely, data on (i) average delay and (ii) critical gap. The data on average delay may be collected the same way as in signalized intersections, as the approximation made there (see Figure 13) is also valid here except for the fact that the cumulative departure line does not follow any fixed pattern (since the departure process is also stochastic).

Data on critical gap is slightly more difficult to obtain in the field. The reason for this is that a single driver may reject a lot of gaps but always accepts only one gap. From this fact one can only say that the critical gap for the person is greater than the largest rejected gap and smaller than the accepted gap. Hence such a method of data collection will only yield ranges as estimates of the critical gaps for different drivers. Further these ranges could be quite large making a reasonably precise estimate of the critical gap almost impossible.

Therefore, often one has to assume that the drivers coming at the intersection are reasonably similar in their gap acceptance behaviour in order to obtain large number of rejected and accepted gaps. A simple determination of the critical gap from the largest rejected and the smallest accepted gaps is not feasible as often the former may be greater than the latter due to differences in gap acceptance behaviour of different drivers. Drew [#!dre1!#] suggests the following procedure to determine the critical gap.

• Observe an unsignalized intersection and obtain data on the largest gap rejected by a driver and the gap accepted by the driver for many drivers.
• Divide the time scale into small intervals (say of 0.5 seconds duration) and determine for every time class, $t$, (say gap size between 2 and 2.5 seconds) the number of such gaps accepted and the number of such gaps rejected.
• From this calculate the number of gaps less than $t$ which are accepted and the number of gaps greater than $t$ which are rejected.
• Plot the cumulative curves on the same graph of ``number of gaps'' versus ``$t$''.
• Report the value of $t$ where these plots intersect as the critical gap. This value of $t$ represents that gap size for which the number of gaps greater than $t$ which have been rejected are equal to the number of gaps smaller than $t$ which have been accepted.