ITE Journal April 2018 - 46
Geometric elements refer to the branch of highway engineering
concerned with the positioning of the physical elements of
roadways according to standards and constraints. The basic
objectives in geometric design are to optimize efficiency and
safety while minimizing cost and environmental damage.4 Figure
1 illustrates general geometric elements at parclo interchange
terminals, including median type, median width, turning radius,
and stop line positioning.
This study identifies possible geometric elements to direct
driver movements at parclo interchange terminals and quantifies
relationships between these geometric elements and WWD
based on crash and field data analysis to help drivers' decision
making and movements, which ultimately improves traffic safety.
Accordingly, this study provides guidelines for parclo interchange
terminal design and countermeasures for WWD prevention.
Of interest is the application of geometric elements to deter WWD
at the intersections of interchange ramps and crossroads. Past
studies about use of geometric elements to reduce wrong-way
entries at interchange terminals are summarized.
Eyler recommended using an appropriate angle for
sweeping connections of exit ramps to crossroads, such as
outer connections, loops, and some diamond ramps, to make
interchange terminals less susceptible to WWD.5 For example, if
left turns from exit ramps are prohibited because of a connecting
one-way roadway or the presence of a raised median on the
roadway, an acute angle should be used to connect exit ramps
Figure 1. Geometric elements at partial cloverleaf interchange terminals.
Ap r i l 2018
i te j o urn al
to the roadway. This is primarily due to the inherent capability
of the formed acute angles with the crossroads, which causes
turning movements in either direction difficult.
Cooner and Ranft evaluated the most effective traditional
and innovative countermeasures throughout the United States
to reduce wrong-way movements.6 They developed a typical
wrong-way crash profile utilizing four years of wrong-way crash
data on Texas freeways. They provided recommended guidelines
for parclo interchange terminals. By adding raised channelizing
islands separating adjacent entrance and exit ramps to reduce
ramp-throat width, this geometric design can discourage
wrong-way entries by making ramps uninviting to drivers,
especially at multilane exit ramps.
Chassande-Mottin and Ganneau proposed to reduce the
complexity of intersections connecting to interchanges for
WWD prevention.7 Specifically, the use of roundabouts instead
of multiple islands can reduce drivers' confusion at interchange
terminals as well as for control access, thereby, reducing
wrong-way entries. Zhou et al. conducted a WWD study to
identify contributing factors and countermeasures to reduce
WWD crashes on Illinois freeways.1 A series of geometric
elements were proposed to be potential countermeasures to
decrease WWD crashes in the vicinity of freeway interchanges,
such as using raised median and channelizing islands,
increasing the median width between exit ramps and entrance
ramps, and reducing the turning radius to parclo interchanges.
The North Texas Tollway Authority (NTTA) investigated the
effects of median modifications to reduce WWD incidents at a