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 1lane roundabout different from an ordinary straight stretch of roadway:

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.
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 1520 mph is lower than about 1100 cars per hour. 
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