Designing Crossing Islands for Speed Control and Intersection Safety on Two-Lane Collectors and Arterials

Abstract

Crossing islands at unsignalized intersections, in addition to their pedestrian crossing safety benefits, can also serve as speed control chicanes by forcing vehicles to make a reverse curve. A method is developed for determining the chicane length (and thus, parking setback) needed for a two-lane road for a given lane width, island width, and target speed, based on models of the relationship between road geometry vehicle path radius, and speed. New data on the speed–radius relationship is presented. The concept of “informal flare” is also introduced; it is a common approach geometry that allows a left-turning vehicle to wait for a gap in opposing traffic without blocking through traffic behind it. Using informal flares can make it possible to prevent left-turn blockage without sacrificing a crossing island for a left-turn lane. Curb continuation lines at median openings are presented as a means to enhance informal flare function. Original data are presented relating informal flare function (the tendency of through vehicles to bypass a waiting left-turner) to a road’s half-width. Geometric analysis shows that intersections with crossing islands can fit on roads with right of way as narrow as 60 ft, and with curb-to-curb width as narrow as 40 ft, while still accommodating turning school buses and bike facilities and preventing left-turn blockage. Various performance measures are used to evaluate intersection geometry, including measures related to through vehicle speed, turning vehicles, and pedestrians. With crossing islands, pedestrian safety with respect to left-turning vehicles is substantially improved as the turning path becomes square to the crosswalks, making the vehicle path more predictable and reducing vehicle speed, conflict area size, and pedestrian exposure distance.