Channelized junction

At a channelized junction, islands or markings set the paths that vehicles take, most often for the right turn.^1,2 It gets a road of its own: a slip lane, kept apart from the through lanes by an island, ideally raised and normally a triangle, carries the turning traffic to the cross street.^2,3,4 There the slip lane may give way, stop, merge or have a signal of its own.^3,5 The islands shrink the area where paths conflict, bring merging traffic in at a smaller angle and give people on foot a place to wait.^1,2

In the United States, simple devices to channel traffic date back to the early 1920s, but most of the important highway channelization projects were designed after 1940.⁶ In 1952 the Highway Research Board published 59 examples of channelized intersections, so that engineers could learn from each other's work;⁶ when it revised the collection in 1962, channelization had become much more widely used.¹ By 1985, experience with "over-channelized" intersections had taught engineers that simplicity matters: islands should be kept to the minimum a location needs.⁷

A slip lane suits a heavy right turn: it raises capacity, saves stops and makes room for large trucks.^3,5 Its higher turning speed, an advantage for drivers, raises concerns for people on foot and on bikes.⁵ Drivers looking for a gap in the cross traffic may miss people on foot coming from the other side, and where the slip lane has no signal, drivers turning at higher speeds may yield to them less.^2,5 A 2024 review for Minnesota found crash studies too few to say how slip lanes compare with ordinary right-turn lanes.⁵ Newer designs bring the slip lane into the cross street at nearly a right angle, so that drivers slow down and see who is crossing.^2,5