Splitter islands

Splitter islands (also called separator islands or median islands) should be provided on all roundabouts, except those with very small diameters at which the splitter island would obstruct the visibility of the central island. Their purpose is to provide shelter for pedestrians (including wheelchairs, bicycles, and baby strollers), assist in controlling speeds, guide traffic into the roundabout, physically separate entering and exiting traffic streams, and deter wrong-way movements. Additionally, splitter islands can be used as a place for mounting signs (see Chapter 7).

The splitter island envelope is formed by the entry and exit curves on a leg, as shown previously in Exhibits 6-24 and 6-25. The total length of the island should generally be at least 15 m (50 ft) to provide sufficient protection for pedestrians and to alert approaching drivers to the roundabout geometry. Additionally, the splitter island should extend beyond the end of the exit curve to prevent exiting traffic from accidentally crossing into the path of approaching traffic.

Exhibit 6-26 shows the minimum dimensions for a splitter island at a singlelane roundabout, including the location of the pedestrian crossing as discussed in Section 6.3.7.

While Exhibit 6-26 provides minimum dimensions for splitter islands, there are benefits to providing larger islands. Increasing the splitter island width results in greater separation between the entering and exiting traffic streams of the same leg and increases the time for approaching drivers to distinguish between exiting and circulating vehicles. In this way, larger splitter islands can help reduce confusion for entering motorists. A recent study by the Queensland Department of Main Roads found that maximizing the width of splitter islands has a significant effect on minimizing entering/circulating vehicle crash rates (2). However, increasing the width of the splitter islands generally requires increasing the inscribed circle diameter. Thus, these safety benefits may be offset by higher construction cost and greater land impacts.

Standard AASHTO guidelines for island design should be followed for the splitter island. This includes using larger nose radii at approach corners to maximize island visibility and offsetting curb lines at the approach ends to create a funneling effect. The funneling treatment also aids in reducing speeds as vehicles approach the roundabout. Exhibit 6-27 shows minimum splitter island nose radii and offset dimensions from the entry and exit traveled ways.