Intersection sight distance

Intersection sight distance is the distance required for a driver without the right of way to perceive and react to the presence of conflicting vehicles. Intersection sight distance is achieved through the establishment of adequate sight lines that allow a driver to see and safely react to potentially conflicting vehicles. At roundabouts, the only locations requiring evaluation of intersection sight distance are the entries.

Intersection sight distance is traditionally measured through the determination of a sight triangle. This triangle is bounded by a length of roadway defining a limit away from the intersection on each of the two conflicting approaches and by a line connecting those two limits. For roundabouts, these “legs” should be assumed to follow the curvature of the roadway, and thus distances should be measured not as straight lines but as distances along the vehicular path.

Intersection sight distance should be measured using an assumed height of driver’s eye of 1,080 mm (3.54 ft) and an assumed height of object of 1,080 mm (3.54 ft) in accordance with the recommendations to be adopted in the next AASHTO “Green Book” (4).

Exhibit 6-32 presents a diagram showing the method for determining intersection sight distance. As can be seen in the exhibit, the sight distance “triangle” has two conflicting approaches that must be checked independently. The following two subsections discuss the calculation of the length of each of the approaching sight limits.

Length of approach leg of sight triangle

The length of the approach leg of the sight triangle should be limited to 15 m (49 ft). British research on sight distance determined that excessive intersection sight distance results in a higher frequency of crashes. This value, consistent with British and French practice, is intended to require vehicles to slow down prior to entering the roundabout, which allows them to focus on the pedestrian crossing prior to entry. If the approach leg of the sight triangle is greater than 15 m (49 ft), it may be advisable to add landscaping to restrict sight distance to the minimum requirements.

Length of conflicting leg of sight triangle

A vehicle approaching an entry to a roundabout faces conflicting vehicles within the circulatory roadway. The length of the conflicting leg is calculated using Equation 6-3:

Two conflicting traffic streams should be checked at each entry:

• Entering stream, comprised of vehicles from the immediate upstream entry. The speed for this movement can be approximated by taking the average of the entry path speed (path with radius R1 from Exhibit 6-12) and the circulating path speed (path with radius R2 from Exhibit 6-12).
• Circulating stream, comprised of vehicles that entered the roundabout prior to the immediate upstream entry. This speed can be approximated by taking the speed of left turning vehicles (path with radius R4 from Exhibit 6-12).

The critical gap for entering the major road is based on the amount of time required for a vehicle to turn right while requiring the conflicting stream vehicle to slow no less than 70 percent of initial speed. This is based on  research on critical gaps at stop-controlled intersections, adjusted for yield-controlled conditions (9). The critical gap value of 6.5 s given in Equation 6-3 is based on the critical gap required for passenger cars, which are assumed to be the most critical design vehicle for intersection sight distance. This assumption holds true for single-unit and combination truck speeds that are at least 10 km/h (6 mph) and 15 to 20 km/h (9 to 12 mph) slower than passenger cars, respectively.

In general, it is recommended to provide no more than the minimum required intersection sight distance on each approach. Excessive intersection sight distance can lead to higher vehicle speeds that reduce the safety of the intersection for all road users (vehicles, bicycles, pedestrians). Landscaping can be effective in restricting sight distance to the minimum requirements.

Note that the stopping sight distance on the circulatory roadway (Exhibit 6-30) and the intersection sight distance to the circulating stream (Exhibit 6-32) imply restrictions on the height of the central island, including landscaping and other objects, within these zones. In the remaining central area of the central island, higher landscaping may serve to break the forward vista for through vehicles, thereby contributing to speed reduction. However, should errant vehicles encroach on the central island, Chapter 7 provides recommended maximum grades on the central island to minimize the probability of the vehicles rolling over, causing serious injury.