All results presented in this section refer to the situation of a 1-lane minor street merging into a 1-lane major street, with the intersection control being a yield sign. Fig. 32.6 (a) shows what happens if the ``reservation'' rule from the Transims production code is no longer used. Clearly, if vehicles from the major road do reserve cells on the outgoing link only if they are actually going there, many more vehicles from the minor lane can make the turn, effectively leading to an ``alternating'' vehicle pattern. This may be desirable in some situations.
Figs. 32.6 (b) shows what happens when one then changes
``accept when
'' to ``accept when
''. This seems like a negligible difference in the rules;
yet, the results are quite different in the congested regime. Whereas
in the first, many vehicles are able to get into the congested major
road, in the second, only few of them make it. The difference is
easiest explained by looking at a vehicle of speed zero on the major
road just in front of the merge point, with space for a vehicle
downstream of the merge point. With the first rule, a vehicle at the
yield sign will accept the move and move in front of the vehicle on
the major road, in the second case, it will not. Both scenarios seem
to be plausible to us; only systematic measurements can probably
resolve which one is better for a simulation model. - Also note that
the rule in (b) generates similar flows as the Transims production
version.
Fig. 32.6 (b), (c) and (d) show the result of different speed
limits (same speed limit for both streets). A high average free speed
of approx. 130 km/h ( mph, generated by
),
maybe a freeway merge, generates a flow of approx. 2000 veh/hour/lane
in the incoming lane when there is no traffic on the major road
(Fig. 32.6 (c)). From there, maximum incoming flow decreases
continuously. Lower average free speeds of approx. 75 km/h (50 mph,
Fig. 32.6 (b)) and 50 km/h (30 mph, Fig. 32.6 (d))
generate lower maximum incoming flows and are generally closer to the
Highway Capacity Manual curve. Yet, it should be clear that, contrary
to the HCM, the ``minor'' flow is also a function of the speed limit
and not only of the gap acceptance (the gap acceptance is the same in
all three simulations).
A last series of experiments shows the effect of different values for
the gap acceptance. Figs. 32.6 (e) and (f) show ``accept when
and
''. Clearly, more vehicles are
accepted, leading to a higher flow of turning vehicles as a function
of the flow on the major road. Note that the flow via the yield sign
is never higher than 1800 minus the flow on the major road. This
reflects the fact that the major road cannot have a higher flow than
1800 veh/h/lane (free speed approx 50 mph); traffic through the yield
sign can thus at most fill the major road to capacity. This explains
why the acceptance of much smaller gaps do not produce a stronger
difference. The situation is clearly different for unprotected turns
across instead of into traffic, as can be seen for the
left turns in the next section.