At-Grade Rail Crossings

Locating any intersection near an at-grade railroad crossing is generally discouraged. However, roundabouts are sometimes used near railroad-highway at-grade crossings. Rail transit, including stations, have successfully been incorporated into the medians of approach roadways to a roundabout, with the tracks passing through the central island. In such situations, the roundabout either operates partially during train passage, or is completely closed to allow the guided vehicles or trains to pass through. The treatment of at-grade rail crossings should follow primarily the recommendations of the Manual on Uniform Traffic Control Devices (MUTCD) (3). Another relevant reference is the FHWA Railroad-Highway Grade Crossing Handbook (4).

There are essentially two ways in which rails can interact with a roundabout, as shown in Exhibit 8-1:

• Through the center; or
• Across one leg in close proximity to the roundabout.

In either case, traffic must not be forced to stop on the tracks. A new intersection should not be designed with railroad tracks passing through the center of it. However, on occasions, the rail line passes through an existing intersection area. The traffic engineer might be faced with a decision whether to change the intersection type to a roundabout or to grade-separate the crossing.

A gated rail crossing through the center of a roundabout can be accommodated in two ways. The first method is to prevent all vehicular traffic from entering the roundabout. The second method is to prevent traffic from crossing the tracks while still allowing some movements to occur. This latter method will have lower delays and queues, but it may be more confusing and less safe.

A gated rail crossing adjacent to a roundabout can be accommodated in two ways, as shown in Exhibit 8-2:

• Method A: Closure only at rail crossing. This method prohibits vehicles from crossing the rails but still allows vehicles to enter and leave the circulatory roadway. This method allows for many of the movements through the roundabout to continue to run free, if a queue does not build to the point of impeding circulation within the roundabout. A queuing analysis should be performed using the expected volume crossing the rails and the expected duration of rail crossing to determine the likelihood that this blockage will occur. In general, this method works better than Method B if there is sufficient separation between the roundabout and the rail crossing. If blockage is anticipated, the designer should choose Method B.
• Method B: Closure at rail crossing and at most entries to the roundabout. This method closes all entries to the roundabout except for the entry nearest the rail crossing. This allows any vehicles in the roundabout to clear prior to the arrival of the train. In addition, a gate needs to be provided on the approach to the rail crossing exiting the roundabout to protect against possible U-turns in the roundabout. This causes increased queuing on all approaches but is generally safer than Method A when there is insufficient storage capacity between the roundabout and rail crossing.