ITE Journal - March 2021 - 39

Courtesy CalTrans and the City through GHD.

at a higher rate of speed and coming from a rural roadway where
the closest adjacent intersection is almost 4 miles (6.4 kilometers)
further east. Additionally, the approach to the intersection
is on a horizontal and vertical curve. Therefore, the longer
splitter is needed to alert the driver to the change in condition
and provide that visual and physical queue that the roadway is
changing ahead. For the west leg (traveling eastbound), drivers
are approaching from a slower and more open environment
with businesses and uncontrolled intersections along the way.
Additionally, there are business access points on both the north
and south side of the intersection. Based on this criteria, the
shorter splitter island was warranted.

Figure 1. State Route 49 /Main Street, Plymouth, CA, USA.

Courtesy MTJ Roundabout Engineering.

Conspicuity on Approach. Approach splitter island/median
landscaping along with central island features such as mounding,
and appropriate selection of landscape vegetation design features
can assist drivers in recognizing the need to reduce their speed and
navigate the intersection safely. The project shown in Figure 2 is an

Courtesy Hillary Isebrands.

before-and-after comparison of crash data showed in the six years
before the roundabouts were constructed, two fatalities and nine
suspected serious injuries occurred. In the 11 years after the rural
roundabouts were constructed, there were no fatalities and seven
suspected severe crashes. This is a 66 percent reduction in fatal and
severe crashes at these rural roundabouts.8
Splitter Island Lengths. Splitter island designs can be extended
approximately to a point where drivers might be expected to
decelerate. A 1993 Austroads guide, referenced in NCHRP 672
Roundabouts: An Informational Guide, suggests a minimum of 200
feet (ft.) (61 meters [m]) for high-speed roadways.9, 10 Furthermore,
NCHRP Report 672 states that good design encourages drivers to
slow prior to the roundabout and suggests designing a comfortable
deceleration with a minimum of 100 ft. (30.5 m) splitter island and
200 ft. or more desirable splitter island length is desirable for higher
speed roadways. Some agencies have adopted a speed-based splitter
island design length approach based on comfortable stopping
distance for the prevailing approach speeds.
Even with this guidance, it is noted that prevailing speeds, the
context, and horizontal and vertical alignments of the specific location
must be taken into consideration in determining appropriate splitter
island lengths to assist with addressing driver expectancy, versus
relying on standardized or specific lengths for all applications. It's
also noted that appropriate horizontal geometric shifts (aka " slight "
chicane) on approaches can be useful to assist in addressing driver
expectancy on higher speed roadways. However, similar to splitter
islands lengths it is not recommended to implement the horizontal
shift (chicane) in all conditions and applications, as this can restrict the
ability to find the best fit and optimized safety for a specific location.
Both tangential/flared/taper (shown in Exhibit 6-69 in NCHRP 672)
approaches and successive curve approach designs on approaches have
a similar safety performance record in the United States.
The project shown in Figure 1 is an example of context for
splitter island length. The east leg of the roundabout has a splitter
island of 250 ft. (76.2 m), where the west leg has a splitter island
of just 100 ft. The reason for the difference in splitter island
lengths on the two approaches of the same road is context. Drivers
approaching from the east (traveling westbound) are approaching

A 2006 Florence, KS, USA rural roundabout US 50/US 77 with a
160 ft.-long splitter island.

Figure 2. USH 18 at Bennett Road Dodgeville WI, USA.
w w w .i t e.or g

M a rch 2021

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ITE Journal - March 2021

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