ITE Journal - August 2020 - 36

Single-lane roundabouts, where all entries and exits are one
lane, have been very successful in terms of safety. Multilane
roundabouts, although they still reduce fatal and injury collisions,
do not always reduce total collisions.3 There are several reasons
for this, including higher traffic volumes, higher speeds, and
more complexity in design and driver comprehension. The most
problematic multilane roundabouts are those where all entries and
exits are two or more lanes.4

Common Crashes at Multilane Roundabouts
There are several types of collisions that can occur at multilane
roundabouts that are not possible at single-lane roundabouts. The two
most common are the "left-turn" crash and the "merge-type" crash.
Both are low-speed sideswipe crashes and are shown in Figure 1.
Usually at a two-lane roundabout, drivers can proceed straight
through from either lane. The left-turn crash (Figure 1, left) occurs
when a driver enters in the right lane and travels more than halfway
around the roundabout, colliding with a driver making a through
movement from the left entry lane.
The merge-type crash (Figure 1, right) happens when a driver
enters in the right lane and proceeds through, colliding with a
driver next to the central island exiting immediately downstream.
Here, "merge" is used in quotations because the movement is not a
true merge, as neither driver changes lanes.

What is a Roundabout?
In April 2016, the Road Commission of Washtenaw County, MI,
USA, circulated an online survey to gain an understanding of
driver perception of roundabouts. The impetus was a multilane
roundabout experiencing a high rate of left-turn and merge-type
crashes. Approximately 4,300 residents responded. The main focus
of the survey was a multiple-choice question asking how one should
enter a multilane roundabout. The responses were:

Figure 1. Common Crash Types at Multilane Roundabouts.
36

Au gust 2020

ite j o u rn al

Figure 2. What is a Roundabout?
	 Wait for an appropriate gap in all circulating traffic, 62 percent.
	 Merge into circulating traffic, 34 percent.
	 Wait for the roundabout to be clear of other traffic, 3 percent.
	 Wait for circulating traffic to stop, 1 percent.
Roundabout design in the United States and Canada is based on
practice in the United Kingdom, where a roundabout is considered
to be one intersection. Proceeding straight through is the equivalent
of a crossing movement. This differs from practice in places like
continental Europe, where a roundabout is considered to be a series
of T-intersections. Proceeding straight through requires a right turn
into the roundabout, and then a right turn to exit (see Figure 2).
Since a roundabout in the United States in considered to be
one intersection, unless lane designation signs and arrows indicate
otherwise, a driver entering in the right lane and traveling more
than halfway around a multilane roundabout is making an illegal
left turn from the right lane. That driver would therefore be at fault
in the event of a collision with someone traveling through in the
left lane, since normally both the left and right lanes can proceed
straight through (see Figure 1, left.) Similarly, a driver entering in the
right lane would be at fault in the event of a collision with someone
in either circulating lane, since that driver entered an intersection
and failed to yield to oncoming traffic (see Figure 1, right).
ITE Journal - August 2020

Table of Contents for the Digital Edition of ITE Journal - August 2020
