ITE Journal - March 2020 - 32

ITE RECOMMENDED PRACTICE - COMPANION PIECE

Research Still Needed!
By Jeff A. Lindley, P.E. (F)

Yellow change and red clearance intervals have been a topic of
research for at least the last 60 years. It would be easy to assume that
we now know all we need to know about the subject and that the
remaining challenge is merely to put what we know into practice.
But there is still much about driver behavior at intersections
during traffic signal changes that we don't know with certainty
or completely understand. During the development of the ITE
Guidelines for Determining Traffic Signal Change and Clearance
Intervals Recommended Practice, current knowledge, research, and
practice in this area was documented, but the following 11 areas of
interest were identified where additional study or new research is
needed to expand the body of knowledge. Research in these areas
would be useful in further refining the concepts and procedures in
the ITE Recommended Practice.
ƒƒ Safety benefits of yellow change and red clearance intervals.
Additional study of driver compliance rates with and their
sensitivity to signal timings set for yellow change and red
clearance intervals across different vehicle types would be
helpful. This work should incorporate left-, through- and
right-turn movements as well as the impact on instances of
red-light running. Additionally, the analysis should employ an
approach that can quantify safety benefits related to fatality and
injury reduction. Supporting analysis incorporating non-motorized modes of pedestrian and bicycle movement would be
beneficial as well.
ƒƒ Impact on driver behavior and safety of yellow change
intervals greater than 5 seconds. It is widely thought that
longer change intervals can lead to unsafe behavior once drivers
are aware of and familiar with them. However, this understanding is very anecdotal in nature and available literature is not
definitive on this issue. Continued research in this area would
be improve the body of knowledge.
ƒƒ Perception-reaction time and deceleration for alerted drivers
for turning movements. Additional data and analysis, for
both right- and left-turning vehicles, of the effect of a planned
choice of movement by an alerted driver on perception-reaction
time and deceleration rate. Similarly, whether information
from countdown pedestrian signal indications affect perception-reaction time and deceleration rate. The effect of different
age groups, vehicle types, and approach speeds on these two
parameters would need to be incorporated. The recent availabil32

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ity of high-resolution driver behavior data sets would add value
to this type of research.
Approach and passage speed variations associated with
different left-turn lane characteristics. Left-turn lanes have a
variety of geometric and operational characteristics potentially
affecting their approach and passage speeds that would benefit
from additional research, including (for example): speed limits
less than 30 miles per hour (mph) (50 kilometers per hour [km/
hr]), turn-lane length, number of lanes, signal phasing, and
movements where U-turns are allowed in addition to left turns
on single- or multi-lane approaches. This research should also
examine the significance of these potential effects and whether
they could be practically applied to change and clearance
interval calculations.
Approach and passage speed variations for different
right-turn lane characteristics. Right-turn lanes have a variety
of geometric and operational characteristics potentially affecting
the approach and passage speeds that would benefit from
additional research, including: driver behaviors, speed limits
less than 30 mph (50 km/hr), turn-lane length, number of lanes,
signal phasing, driveway access, and conflicting bicycles and
pedestrians. While characteristics of right-turns are analogous
to left turns, how they affect application of the calculations may
be different. This research should also examine the significance
of these potential effects and whether they could be practically
applied to change and clearance interval calculations.
Passage speed variation on the path through an intersection
from left or right-turns. The approach to estimating the
passage speed for a turning path through an intersection in
the ITE Recommended Practice is based on the 85th percentile
intersection entry speed. Additional empirical analysis of field
data in comparison to theoretical values for small radii and the
curvature of complex paths, along with guidance for application,
would enhance understanding of these relationships.
Data collection methods for approach speeds of through
movements compared to posted speed limits. With the
expansion of automated traffic signal performance measures
programs, the ability to collect and archive intersection
detection data, including vehicle speeds, is rapidly increasing.
Supporting research would examine processes to use data
from detector infrastructure to provide an expanded data set

Shutterstock/RomanYa

Traffic Signal Change and Clearance Intervals:



ITE Journal - March 2020

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