The (lack of) safety or risk of an intersection is generally expressed in the number of crashes per number of crossing vehicles (volume).
Urban intersections
Janssen (2004) calculated risk values of different types of urban intersections in a comparative study. The study indicated a lower risk at:
- roundabouts compared with intersections;
- three-arm intersections compared with four-arm intersections;
- intersections without traffic lights compared with intersections with traffic lights;
- intersections without right of way compared with intersections with right of way;
- intersections without cycling facilities compared with intersections with a separate bicycle path.
The last finding seems to be counterintuitive. However, research has been conducted into the main features of roundabouts only, without looking at the other features of the intersections. Some intersections, for example, require bicycle facilities, whereas other intersections do not. Nor was account taken of the structure and function of the road network the intersections were part of. Therefore, the Janssen study cannot and should not be used to convert ‘unsafe’ intersection types into intersections with a ‘safer’ rating. Only before-and-after studies can establish whether a change in the situation has actually improved road safety.
Jansen (2004) also examined the relationship between injury crashes and traffic volumes at intersections of through-roads with a 50 km/h speed limit. Figure 1 shows that the number of injury crashes usually increases with increasing traffic volume; the exact relationship differs for different types of three-arm and four-arm intersections.

Figure 1. Relationship between number of injury crashes and traffic volume on three and four-arm intersections of urban through-roads with a 50 km/h speed limit. The # sign indicates the number of studied intersections of that particular type (Janssen, 2004).
Rural intersections
Beenker (2004) carried out a before-and-after study into the layout of 60 km/h zones. The study showed that the number of casualty crashes at intersections in these zones had decreased by 47% compared with the situation before the 60 km/h layout was introduced.
A comparative study at network level showed that roundabouts and staggered intersections are safer than four-arm intersections (Hummel, 2001). A staggered intersection is a four-arm intersection which is divided into two three-arm intersections. This prevents serious transverse conflicts. Hummel based his research on data from the United States and several European countries. An Australian before-and-after study by Corben et al. (2007) showed that the construction of staggered intersections resulted in a 93% decline in the number of serious crashes. However, according to Corben et al. staggered intersections do not entirely fit into a safe traffic system because they are not sufficiently forgiving in the case of driver errors, particularly in situations with high speed limits. While transverse conflicts are avoided, other potential conflicts remain.
In Finland the safety of three-arm and four-arm intersections in rural areas was investigated in a before-and-after study (Kulmala, 1995). This showed that 1.3 to 1.4 times more crashes occur on four-arm intersections than on three-arm intersections. Other international studies (including comparative studies) also showed that four-arm intersections are less safe than three-arm intersections and that roundabouts are safer than 'ordinary' intersections (Elvik & Vaa, 2004; O’Cinneide & Troutbeck, 1995).