Fact

Roundabouts - What road safety effect do roundabouts have?

A roundabout reduces the number of potential conflicts at an intersection and ensures that, in addition to less serious rear-end conflicts, only one (lateral) conflict type remains (see Figure 2). A roundabout also reduces the approach speed of traffic. The correct approach speed is approximately 30 km/h because in many cases crashes between motor vehicles and pedestrians or cyclists end without fatal injury at this speed (Wegman & Aarts, 2006). However, in order to guarantee this approach speed the roundabout should meet the specific design requirements regarding the consecutive  bends that motor vehicles have to follow when approaching and driving on a roundabout. The current requirements (CROW, 1998) use an approach speed of about 35 km/h. CROW (1998) provides arithmetical directions for designing a roundabout with lower approach speeds.

Figure 2. Potential conflicts on various intersection types.

 

Urban roads

Various studies have been carried out in the Netherlands about the safety effects of replacing an intersection by a single-lane roundabout (Van Minnen, 1990; 1995; 1998; Dijkstra, 2005). Churchill et al. (2010) evaluated the effects of approximately 2000 roundabouts that were constructed during the period 1999-2005. They found a 76% reduction of the number of fatalities and a 46% reduction of the number of serious casualties (fatalities and serious road injuries). These reductions are smaller than those found by Van Minnen (1998). The roundabouts in the latter study, however, had been constructed much earlier. These roundabouts probably replaced intersections that were much more dangerous than those that were replaced by roundabouts that were constructed during the more recent period.

Elvik (2003) studied 30 before-and-after studies about roundabouts from various countries, made corrections, and found an average net effect of 30-50%.

Rural roads

Fortuijn (2005a) studied the effects of replacing 58 intersections by 51 single-lane roundabouts and seven turbo-roundabouts on provincial roads in the province of Zuid-Holland. All roundabouts had separate bicycle tracks and had no priority for cyclists. The study showed a decrease in the number of injury crashes (including those with slight injury) of more than 80% on both single-lane and two-lane roundabouts. After correcting for the 'diminishing safety returns' described above, the percentage decreases to 70%.

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