Their top is wider, typically 12 feet or more, enabling both the front and back wheels of a standard vehicle to be on the table at the same time. They can be traversed comfortably at 15-25 mph, a little faster than speed humps, and they too can cause increases in traffic noise from braking and acceleration. Traffic Circles Traffic circles force drivers to slow down by exerting lateral motion on vehicles as they go around the circle. The circle must be at least 12 feet across to be effective. Usually, they are formed around a raised, landscaped island.
Circles are most effective at reducing left-turn and sideways collisions.
Singly, they only have a short-term effect on traffic speed, since drivers tend to reaccelerate quickly after passing through. However, if a series of them is constructed, they can noticeably slow traffic. There are no United States DOT guidelines for roundabouts, but they are generally no smaller than 50 and no larger than 115 feet in diameter. Roundabouts can calm traffic indirectly by changing the character of a roadway as it goes from a busier, commercial district to a residential one. Their landscaping can also help to calm traffic.