I'm observing less delay in Synchro and SimTraffic than I expect for permitted U-turns (in a shared lane with left turns) at a signal on a high-speed divided highway. In SimTraffic, I've spent some time observing the behavior of the U-turning vehicles and the conflicting through vehicles that they're turning in front of. The conflicting through vehicles almost always appear to decelerate, often quite rapidly, as a courtesy to the U-turners. (The intersection is not congested).

I think I understand how the gap acceptance factor and courtesy decel rate for different driver types factor into this, but I'm still concerned that there seems to be no reaction time between the entry of a U-turner into the intersection and the corresponding courtesy deceleration by the conflicting through. Advancing frame-by-frame through my simulation, I can see the conflicting vehicle experience a deceleration of -8 (courtesy) or -12 (max) ft/s2 at the instant that the U-turner begins to accelerate into the intersection. Any way to remedy this?

By the way, does Synchro handle permitted U-turns any differently than permitted lefts for its calculations and optimizations? I can't find anything to that effect in either the Help docs or the HCM.

Thanks,
Josh

Hi Josh,

It is possible that a vehicle will slow for a U-turn. Actually, what is probably happening is that the U-turn has been approved to enter the intersection. It then enters, makes the turn and accelerates. This may cause the through vehicle to slow down for the accelerating the vehicle.

You can reduce the gap acceptance factor and the courtesy decel for the drivers. You might note what type of driver is allowing the U-turns in and what type of U-turner it is (conservative type 1 or agressive type 10).

On the Synchro question, the U-turns are handled like left turns. The user can over-ride the left turn factor or the saturation flow if necessary.

Thanks,

John.

PS - Feel free to email us the file if you think there is an issue with it.

John Albeck
Trafficware