Roundabouts: capacity - 1

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Let's adapt the simple model developed earlier to estimate the capacity of a typical roundabout. There are two important properties that make a 1-lane roundabout different from an ordinary straight stretch of roadway:

  1. curvature
  2. merging traffic


To account for curvature, we will use in the model a vehicle speed that is limited to the circulating speed to be expected, limited by a reasonable limit on the centripital acceleration. This neglects some smaller, but probably significant effects, such as an increase in safe stopping distance if the vehicle is simultaneously turning.

Merging creates many kinds of delaying effects. Here are a few examples.

  • A driver who needs to merge must wait for a gap of the required size (referred to in the industry as a "critical gap").
  • A driver traversing the roundabout in heavy traffic has chosen his speed and follow-up separation ("headway") as aggressively as he deems safe. Now, a car merges into that space. He has the choice of accepting greater risk, or slowing down. I think slowing down is the healthier and usual USA driver's choice. I would call this "merge friction".
  • A driver who is waiting to merge must judge times and distances, but these are partially affected by (unknowable) decisions of drivers who are approaching, circulating, or exiting the roundabout. I would call this "fuzzy merge conditions".
  • A large bus or truck is longer, accelerates more slowly, and negotiates curves more slowly. Similar additional delay is introduced by a timid driver.

Thus, determining the effects of merging is complicated. Even the best roundabout models account for these effects only crudely and partially.

In the simple model being used here, I will ignore merging effects completely!

Caveat: This makes the simple model application to roundabouts significantly optimistic compared with the real world.

From this discussion, we expect that the entry capacity of a typical single lane roundabout at 15-20 mph is lower than about 1100 cars per hour.

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