ITE Journal January 2018 - 47

is how do the characteristics of the following vehicle, specifically
following headways and vehicle classification, influence the behavior
of the lead vehicle? The phrase, "You must stop if it is safe to do so"
appears in 11 driver training manuals. This language is ambiguous
in defining unsafe stopping conditions. One possible interpretation
of an unsafe stopping choice is related to the likelihood of a rear-end
collision. The likelihood of a rear-end collision is influenced by
the headway and classification of the lead and following vehicles
involved. These two issues are considered in the following cases.
Case 1: Both the lead and follow vehicles at the CY onset are
passenger cars. Most drivers will proceed through the intersection
when within 2.5 seconds of the stop line.13 Headway, speed, and
perception reaction time (PRT) can play a vital role in the driver
decision making of the following vehicle. Drivers may be hesitant
to stop at the CY onset when closely followed due to increased
rear-end crash risk. Allsop et al. indicated that drivers of following
vehicles were more likely to be RLR at the CY onset when the
headway of following vehicles was less than 2 seconds.19
Case 2: A heavy vehicle (HV) following a passenger car at the
CY onset. According to Gates and Noyce (2010), vehicle type has a
significant effect on the rate of deceleration and RLR.8 HVs behave
differently at the CY onset due to characteristics of the vehicle and
driver. The difference in driver behavior of passenger cars and HVs
(i.e., single unit trucks, recreational vehicles, buses, and semi-trailers) has been previously investigated.20 HVs were less likely to stop
when the CY was displayed and they were more likely to perpetrate
RLR. Gates and Noyce (2010) state that HVs cannot stop as rapidly
as passenger cars and the operational cost of HVs is higher when
delayed. Also, the deceleration rate of HVs in dilemma zones has
been shown to be lower than passenger cars when stopping.21

Results
The classification of guidance provided to drivers in driver training
manuals indicate that six states provide Class 0 (do not fit any of
three classes), two states provide Class 1 guidance, six states provide
Class 2 guidance, and 36 states provide Class 3 guidance (Figure 6 A).
However, the findings from NCHRP Report 731 analysis of yellow

light laws classified 37 states as Class 1, four states as Class 2, and
nine states as Class 3 (Figure 6 B). Table 1 illustrates the comparison
between NCHRP Report 731 and DTM Classifications. Based on this
classification scheme, language that appears in state laws and driver
training manuals may lack the needed degree of specificity.
Table 1. Comparison between NCHRP Report 731 and driver training
manuals classifications.
NCHRP Report 731

Driver Manual Language

Class 1

Class 2

Class 3

Class 0

Class 1

Class 2

Class 3

74%

18%

12%

72%

For example, the 2015 Montana Driver Training Manual states
that, "A steady yellow signal means "CAUTION." Cautiously enter
the intersection. The signal is about to turn red. Do not enter an
intersection against a steady yellow light unless you are too close to
stop safely." This driver training manual guidance should be classified
as a Class 3. The Montana yellow law indicates that, "Vehicular traffic
facing a steady circular yellow or yellow arrow signal is warned
that the traffic movement permitted by the related green signal is
being terminated or that a red signal will be exhibited immediately
thereafter. Vehicular traffic may not enter the intersection when
the red signal is exhibited after the yellow signal."5 The language in
the Montana yellow law should be classified as Class 1. The slight
variations in language may cause confusion regarding what the
correct action should be at the onset of the yellow indication.
Our classification shows that only 4 percent of states follow Class
1 guidance in driver training manuals, but the vast majority of state
laws (74 percent) are classified as Class 1 in NCHRP Report 731. The
results also indicate that 12 percent of states follow Class 2 guidance
in DTMs and 8 percent of state laws classified as Class 2 in NCHRP
Report 731. The large percentage of states (72 percent) follow Class
3 guidance in DTMs and 18 percent of state laws were classified
as Class 3 in NCHRP Report 731. The results indicate an apparent
inconsistency between the DTMs and yellow state laws (Figure 6).

Figure 6. Classification of yellow light guidance in driver training manuals (A) and state laws (B)
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Ja nu ar y 2018

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Table of Contents for the Digital Edition of ITE Journal January 2018