Collision types

It is instructive for designers to examine details of collision types and location at roundabouts. Statistics are available for roundabouts designed according to local practices in France, Queensland (Australia), and the United Kingdom. It should be noted that the reported frequencies are to some extent related to the specific design standards and reporting processes used in these countries.

Exhibit 5-12 presents a summary of the percentage of crashes by collision type. The numbered items in the list correspond to the numbers indicated on the diagrams given in Exhibit 5-13 as reported in France. The French data illustrate collision types for a sample of 202 injury crashes from 179 urban and suburban roundabouts in France for the period 1984–1988 (12). For comparison purposes, data from Queensland, Australia (13) and the United Kingdom (1) have been superimposed onto the same classification system.

The results in Exhibit 5-12 are instructive for a number of reasons:

• A variety of collision types can take place at roundabouts. A designer should be aware of these collision types when making decisions about alignment and location of fixed objects. It is recommended that these collision types be adopted as conflict types in the U.S. to conduct traffic conflict analysis and report crashes at roundabouts.
• Although reporting methodologies may vary somewhat, crash experience varies from country to country. This may be due to a combination of differences in driver behavior, and design features.

Three of the predominant types of collision are: (1) failures to yield at entry to circulating vehicles, (2) single vehicle run-off the circulatory roadway, and (3) single vehicle run-into the central island. A more recent crash study (14) confirmed a high proportion of single vehicle crashes: 49 percent in rural areas, versus 21 percent in urban areas. According to crash models from the United Kingdom, single vehicle crashes range between 20 and 40 percent depending on traffic and design characteristics of sites. In the United Kingdom models, separation by urban and rural areas is not provided.

To reduce the severity of single vehicle crashes, special attention should be accorded to improving visibility and avoiding or removing any hard obstacles on the central island and splitter islands in both urban and rural environments. A French study (14) identified a number of major obstacles that caused fatalities and injuries: trees, guardrail, concrete barriers, fences, walls, piers, sign or light poles, landscaping pots or hard decorative objects, and steep cross-slopes on the central island.

In rural areas, the benefit of lighting has not yet been quantified. In France, only 36 percent of the rural sites are lighted. At these sites, 46 percent of all crashes, and 49 percent of single vehicle crashes occur at night (14).

The French study (7) in 15 towns of 202 urban roundabout crashes compared with all crossroads reported the percentage of crashes by user type, as shown in Exhibit 5-14. The percentage of crashes concerning pedestrians was similar to all crossroads. However, the percentage of crashes involving bicycles and mopeds was larger—15.4 percent for urban crossroads overall versus 24.2 percent for roundabouts, i.e., almost 60 percent more.