Validation of rules for unprotected turns

The typical measurement for unprotected turns is the maximum incoming flow rate as a function of the flow on the priority street. Such plots look like those in Fig. 32.6 with flow on the minor road (y-axis) as function of flow on the major road (x-axis). For interpretation, best start in the top left corner. Since there is no flow on the major road, flow from the minor road can enter at a high rate. With increasing flow on the major road, flow from the minor road is reduced. When the major road reaches capacity, the flow from the minor road is nearly zero. When the density on the major road goes above the maximum-flow density, then the flow on the major road is again reduced, but this time by congestion. In Fig. 17.9, vehicles from the minor road still have a hard time entering. In contrast, the gap acceptance rule from Fig. 17.9 allows vehicles from the minor road to enter into the major road under congested conditions, effectively modelling a ``zipping'' effect.

Two important messages are:

• Seemingly small changes, such as the change of gap acceptance from ``$>$'' to ``$\ge$'', can have large consequences. Such small changes can also easily be caused by the actual implementation of the rules. For example, in the Transims micro-simulation traffic on the major street reserves cells on the outgoing link, even if in the end the vehicle does not claim it. This clearly reduces opportunities for vehicles from the minor road.

• Further details need to be taken from local conditions. For example, the flow from the minor into the major road when there is no traffic on the major road depends on speed limits and intersection layout, such as the curvature of the turn. This situation will rarely occur in reality, since if there is traffic on the minor road, there is usually also traffic on the major road. Exceptions are situations such as the end of soccer games or evacuation scenarios.

  Similarly, there are differences between yield and stop, and if the traffic from the minor street merges with the traffic from the major street, or crosses. Again, although the tendency of these changes are clear, exact flow values need to be taken from local conditions.

Figure 17.7: Two different rules for the case of a 1-lane minor road controlled by a yield sign merging into a 1-lane major road. (a) Acceptance rule ``accept if $gap > 3 \cdot v_{oncoming}$''. $v_{max}=3$. (b) Acceptance rule ``accept if $gap \ge 3 \cdot v_{oncoming}$''. Note that this seemingly small difference has a strong effect on throughput in the congested situation. (a) models that vehicles from the minor road cannot enter the major road once the major road is congested; (b) essentially models a ``zipping'' behavior, i.e. that vehicles from the major and the minor road alternate once the major road is congested.

[] []