What I am doing is creating a roundabout simulation, and it is working pretty good, but there is one more parameter that I would like to see go a litle farther than is allowed: gap acceptance. I believe that in a roundabout the gap acceptance factor should be lower because of the 15 mph speeds of vehicles in the roundabout, and because it is therefore a little easier to "jump in" to that traffic stream when there is even a fairly "small" gap (relatively speaking compared to a four way intersection). Any suggestions?

I have already tried modifying the driver parameters in Simtraffic to make all 10 vehicle types "aggressive", which really doesn't look too aggressive in a roundabout, if you know what I mean. At a signalized intersection, I can see why it would be aggressive to jump into a stream of traffic moving at faster speeds, but not at a roundabout where "cross traffic" speeds are low and traveling in a circular path as well. A sideswipe is about the worst potential accident, and so I think that the gap acceptance factor should be lower than the lowest 0.75 allowed in the software. If that were the case, I could get my results to match better what is happening in software like Arcady and Rodel. Thanks for your help!

Hi Grant,

Are you modeling the roundabout as a series of 4 unsignalized intersections? I am assuming this is how you are simulating this. As you know, SimTraffic version 5 really isn't designed to model a roundabout. So, the gap acceptance logic would be designed for traditional style intersections. So, if you have reduced the gap acceptance factors to the lowest values in the SimTraffic Driver Parameters, then you will have reached the limit. You might increase the turning speed of the vehilces that are entering, and reduce the link speeds on the links within the "roundabout".

As you might know, we are looking into these types of things for future versions of the software. This type of behavior would require a change in the logic of drivers entering a traffic stream.

Thanks,

John.

John Albeck
Trafficware

John:

Thanks for the tip.

The right turn speed in the LANES window did the trick. It is the parameter that allows me to get those vehicles in the "roundabout" without creating a stack of vehicles a quarter mile long waiting to get in.

I know it probably sounds strange, but the best way to model this roundabout (which is a single intersection expanded to a 170' diameter roundabout) was with 9 nodes, and a separate link for each entry approach and a link for each exit path (total of 8 links). It looks pretty cool in SimTraffic with all those vehicles moving on each approach. Look forward to your software updates in the future.

Grant Johnson, PTOE
PRISM Engineering
www.prismworld.com