I can't get SimTraffic to properly animate two approach lanes into my roundabout...

I have a four-leg roundabout with all movements permitted, with two entrance / two exit lanes EB and WB, and single entrance / single exit lanes NB and SB. The EB/WB approach lanes are coded as THRU+THRU/RIGHT; the NB/SB approach lanes are coded as a single THRU.

However, in SimTraffic, EB/WB traffic going straight "thru" the roundabout will not use the inside approach lane. All the approaching traffic merges into the outside lane, travels through the outside roundabout lane, and then spills back into both receiving lanes. Only the "left" turns use the inside lanes approaching the roundabout, and within the roundabout itself. I've double-checked, and there are no major "lane-change distance" issues downstream that would influence my lane-utilization to the degree that I'm seeing. What am I missing that is forcing all thru-traffic into the outside lane only?

Thanks - Chad.

Hi Chad,

For the EB and WB approaches, you'll want to code the Two Lane Exit to "Yes". This is found in the Signing window and will allow the center lane of the roundabout to exit the circle.

Thanks,

John.

John,

I already have the EB/WB approaches set with the Two Lane Exit as "Yes". Not that easy...the problem lies elsewhere. Any other ideas?

Thanks - Chad.

Hi Chad,

It then might have something to do with positioning distances and advantages. On the downstream end, are any of the lanes dropping or becoming turn lanes? If so, vehicles may favor one lane over the other due to what they are doing downstream. On the link with the lane drop or turn lane, you could try reducing the Positioning and Mandatory distances. If that doesn't seem to explain it, please feel free to email us your Synchro file (support@trafficware.com).

Thanks,

John.

There are no immediate downstream lane drops, so I don't think lane change distance is a factor. To make sure, I cut the intersection out of the model and let it run by itself with both lanes free-flow off the end of my network. Same problem - no through traffic will use the inside roundabout lanes.

I'll e-mail the file to you directly. Thanks in advance for your help.

John,

Thanks for your e-mail reply. As you mentioned in that e-mail, having an inside radius less than 21' or so does seem to impact the lane utilization. However, after I increased our inside radius, I further noticed that it may not be the only culprit. It also appears that the difference between the inside radius and outside radius values also affect the lane utilization through the roundabout in SimTraffic.

Anyway, thanks for your help. Our two-lane roundabout now seems to be running more like it should.

Thanks again - Chad.

Hi,

I pretty much have the opposite problem:

In my two-lane roundabout (which is an isolated model), cars only use the outside lane for thru-movements. But only in EB direction. WB works fine.

Quite strange...