ARCADY 7 onwards provide options for setting up entry and exit restrictions on arms, which can be used to model bottlenecks directly. The advice below is aimed at users of older versions of ARCADY.
Although a Zebra crossing doesn’t have exactly the same effect as any other bottleneck on a vehicle-by-vehicle basis, over the longer term it should be a good proxy.
The formula for the vehicular capacity of a Zebra crossing, as used in ARC/PIC, was first derived by J. D. Griffiths (Transportation Science, Vol 15, No. 3, August 1981). By using this formula (see below) you can fabricate a dummy Zebra crossing with any vehicular capacity you wish (i.e. that of the bottleneck) – the most obvious way of doing this is to adjust the pedestrian flow rate. If you are going to use this technique, it is advisable to create a spreadsheet based on this formula, particularly given the units.
C is vehicular capacity of Zebra crossing (Vehicles/Second)
U is two-way pedestrian flow on crossing (Peds/Second)
A is mean crossing time of pedestrian (Seconds)
(ARC/PIC assume a mean crossing speed of 1.4 metres/second)
B is mean time headway of vehicles when there are no pedestrians crossing (Seconds) (the reciprocal of vehicles per second saturation flow)
e = 2.7183 (to four decimal places)
One problem is that the bottleneck generally applies to traffic in one direction only. For example, if you insert a dummy Zebra crossing on one arm of a roundabout because of an exit restriction, you will also affect the entry capacity on that arm. Fortunately there is an easy solution:
Model the dummy Zebra as a two-stage crossing with central refuge. Make the distance between the crossing and the junction exit equal to the true value, but the distance between the crossing and junction entry equal to some huge number. In that way, the program assumes that the Zebra is a very long way from the junction entry, and therefore has virtually no effect on entry capacity.
Please note that this suggested solution has not been subjected to the same rigorous validation that is associated with those features built into ARCADY itself.
(updated Oct 2008)