Unprotected turns

Somewhat more difficult are unprotected turns, i.e. turns that are not regulated by traffic signals and where vehicles need to merge on their own without accidents. Typical examples of this are yield, stop, ``right on red'', left turns against oncoming traffic, and on-ramps to freeways. The mechanism here is again a ``gap acceptance'' similar to the safety criterion (S) for lane changes (Fig. 17.6). That is, the vehicle on the incoming road moves into the major road if the gap there is big enough. This gap stretches upstream, since the incoming driver does not want the car upstream on the major road to crash into him/herself. The standard reference for highway engineers, the Highway Capacity Manual (113) states that drivers accept gaps that correspond to time headways of approximately 5 seconds or more, which means that the spatial gap needs to be proportional to the speed of the oncoming car (Fig. 17.6). In our standard CA implementation, this would mean that the accepted gap would have to be at least five times the oncoming vehicle's velocity. When implementing this rule, it turns out that a factor of three instead of five gives much more realistic flow rates (88). It is not totally clear why this is the case.

[[say something about merges/weaving. integration refs?]]

Figure 17.6: Illustration of gap acceptance for a left turn against oncoming traffic. From Nagel et al. (88).