Approach curves
Roundabouts on high-speed roads (speeds of 80 km/h [50 mph] or higher), despite extra signing efforts, may not be expected by approaching drivers, resulting in erratic behavior and an increase in single-vehicle crashes. Good design encourages drivers to slow down before reaching the roundabout, and this can be most effectively achieved through a combination of geometric design and other design treatments (see Chapter 7). Where approach speeds are high, speed consistency on the approach needs to be addressed to avoid forcing all of the reduction in speed to be completed through the curvature at the roundabout.
The radius of an approach curve (and subsequent vehicular speeds) has a direct impact on the frequency of crashes at a roundabout. A study in Queensland, Australia, has shown that decreasing the radius of an approach curve generally decreases the approaching rear-end vehicle crash rate and the entering-circulating and exiting-circulating vehicle crash rates (see Chapter 5). On the other hand, decreasing the radius of an approach curve may increase the single-vehicle crash rate on the curve, particularly when the required side-friction for the vehicle to maintain its path is too high. This may encourage drivers to cut across lanes and increase sideswipe crash rates on the approach curve (2).
One method to achieve speed reduction that reduces crashes at the roundabout while minimizing single-vehicle crashes is the use of successive curves on approaches. The study in Queensland, Australia, found that by limiting the change in 85th-percentile speed on successive geometric elements to 20 km/h (12 mph), the crash rate was reduced. It was found that the use of successive reverse curves prior to the roundabout approach curve reduced the single-vehicle crash rate and the sideswipe crash rate on the approach. It is recommended that approach speeds immediately prior to the entry curves of the roundabout be limited to 60 km/h (37 mph) to minimize high-speed rear-end and entering-circulating vehicle crashes.
Exhibit 6-49 shows a typical rural roundabout design with a succession of three curves prior to the yield line. As shown in the exhibit, these approach curves should be successively smaller radii in order to minimize the reduction in design speed between successive curves. The aforementioned Queensland study found that shifting the approaching roadway laterally by 7 m (23 ft) usually enables adequate curvature to be obtained while keeping the curve lengths to a minimum. If the lateral shift is too small, drivers are more likely to cut into the adjacent lane (2).
Equations 6-4 and 6-5 can be used to estimate the operating speed of two-lane rural roads as a function of degree of curvature. Equation 6-6 can be used similarly for four-lane rural roads (13).
Two-lane rural roads:
Four-lane rural roads:
