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

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.