ITE Journal - August 2020 - 37

The Washtenaw County survey result implies that about a
third of the driving population thinks a roundabout is a series of
T-intersections (Figure 2, right). Or that entering a roundabout is
similar to navigating a channelized right turn or freeway on-ramp.
In both cases, the driver only needs to check for a gap in the nearest
lane before entering the traffic stream. Although not an issue at
single-lane roundabouts, this difference between perception and
design practice is a problem at multilane roundabouts in the United
States and Canada.

Possible Countermeasures
There are a number of ways to counter this perception. The
most effective reduces the number of lanes in a roundabout. A
roundabout where all entries and exits are two lanes (a so-called
2x2 roundabout) is shown in Figure 3 (top left). Depending on
traffic volumes and number of lanes on the approaches, the
following configurations could be implemented to lessen the chance
of a left-turn or merge-type crash:
ƒ	 A partial two-lane roundabout (or 2x1 roundabout), i.e.
two-lane entries and exits east-west, single-lane entries and
exits north-south (top right).
ƒ	 A lane configuration such that a roundabout has two-lane
entries but single-lane exits (bottom left).
ƒ	 A turbo roundabout having raised lane dividers to more
definitively convey lane use. The example shown is a "rotor"
type turbo roundabout, which has two-lane exits (bottom
right). This configuration is shown because it would serve as
a replacement for a 2x2 roundabout (top left) at the intersection of two four-lane roads.
Some possible non-geometric countermeasures include:
ƒ	 Special signage (to address the merge-type crash). This is
discussed in the next section.
ƒ	 Repeated sets of lane use signs and arrows, possibly overhead
(to address the left-turn crash).
ƒ	 Not installing, or removing, the circulatory road lane lines.
ƒ	 More public education.
Lane lines in the circulatory road tend to be the exception
rather than the rule in countries like the United Kingdom and
Australia. They have become commonplace at roundabouts in the
United States, although designs in the 1990s and early 2000s did
not have them and, interestingly, did not seem to have a prevalence
of merge-type crashes. Normally lane lines are not carried through
other intersections except to aid certain movements, such as
the line extensions placed for double left turns. Their absence at
a roundabout may subtly convey the message that it is a single
intersection, and cause drivers to hesitate slightly before entering
because they have more difficultly determining which "lane"
circulating traffic is using.

Figure 3. Possible Geometric Countermeasures.

Sign Effectiveness Testing
The possibility that the merge-type conflict can be mitigated
through special signage was investigated through sign effectiveness
testing carried out with members of the public. The procedure used
follows from work undertaken by Hanscom.5, 6 The testing consisted
of evaluating the Yield sign plaques shown in Figure 4.
A series of laboratory conditions were created to test the two
countermeasures, plus a case where neither is employed. In order to
develop a complete experimental design, the testing also had to
show both cases of an approaching driver (left lane or right lane)
and all three possible cases of circulating traffic (none, inside lane
and outside lane). This resulted in 18 test conditions.
Three stimulus slides were developed for each test condition: 1)
an approach to a two-lane roundabout, 2) just upstream of the entry
crosswalk, and 3) at the yield line looking left. An example sequence
of slides for one of the test cases is shown as Figure 5.

Figure 4. Tested Yield Sign Plaques.
w w w .i t e.or g

Augu st 2020

37


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ITE Journal - August 2020

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