A four-way intersection asks drivers to cross each other’s paths. A roundabout does not. That one change in geometry is why the Federal Highway Administration lists the roundabout among its proven safety countermeasures.
The same crossing of two streets, built two ways. Each dot is a place where two vehicle paths meet.
Try it on an intersection you know
10 is an example, not an average. No national average exists for a single intersection, because the count depends on traffic. Replace it with the real figure from your city or state crash records.
No crash count? Estimate one from traffic
Conflict point counts are for single-lane approaches: 32 at a four-way intersection and 8 at a single-lane roundabout, per the Federal Highway Administration. The estimate applies the Highway Safety Manual figures the agency publishes for converting an intersection to a roundabout: 82 percent fewer fatal and injury crashes from a two-way stop, and 78 percent fewer from a signal. The traffic-based starting figure uses the Highway Safety Manual prediction for multiple-vehicle fatal and injury crashes at urban and suburban four-way intersections, as reprinted in state research reports. It leaves out single-vehicle and pedestrian crashes and applies no state calibration factor, so read it as a typical figure for that much traffic. The reduction percentages are averages across many sites. A real intersection can do better or worse, and a short crash history is a noisy starting point. Planning aid, not an engineering study.
Why it works
- No crossing paths. The right-angle and left-turn collisions that cause the worst injuries have nowhere to happen. What is left are low-angle merges and exits.
- Low speed. The curve brings every vehicle down to under 30 mph, according to the Federal Highway Administration.
- Nothing to run. There is no red light and no stop sign to miss at speed.
What the studies found
- The Highway Safety Manual figures published by the Federal Highway Administration: 82 percent fewer fatal and injury crashes when a two-way stop becomes a roundabout, and 78 percent fewer when a signal does.
- A study of 24 U.S. intersections converted from stop signs and signals found 38 percent fewer crashes of all kinds, 76 percent fewer injury crashes, and about 90 percent fewer fatal and incapacitating crashes (Retting, Persaud, Garder and Lord, American Journal of Public Health, 2001).
Where the evidence is thinner
- Minor crashes. Fender-benders fall much less than injury crashes, and can rise. A Wisconsin evaluation found injury crashes down 38 percent after conversion while total crashes rose 12 percent, mostly property damage at multilane sites. The severe crashes are what go away.
- Multilane roundabouts. The figures here are for single-lane designs. Larger roundabouts are more complicated and the tool does not cover them.
- Walking and cycling. The headline figures count vehicle crashes. People on foot and on bikes need their own look at any specific design.
Where parking comes in
The connection is at the entrance. A roundabout can handle the turn into a garage or campus without a signal, and it gives drivers who miss the entry a way to come back around. It also takes land at the corners, and that land is often a parking lot. For the driveway side of the same idea, see the conflict point calculator and the stroad entry.
Sources: Federal Highway Administration, Proven Safety Countermeasures: Roundabouts (FHWA-SA-21-042), citing the AASHTO Highway Safety Manual; Federal Highway Administration, Roundabouts: An Informational Guide (FHWA-RD-00-067) and Signalized Intersections: Informational Guide; Retting and others, Crash and injury reduction following installation of roundabouts in the United States, American Journal of Public Health, 2001; University of Wisconsin TOPS Laboratory, Safety Evaluation of Wisconsin Roundabouts: Phase 2, 2013.