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their problems with curves. There was also significant vari- Michigan. The two-way, two-lane roads that composed the
ance in how respondents think that speed advisories should route were a mixture of a state-numbered highway and county
be set, from lower than most would go (which is the case roads, although the latter predominated. There were 43 curves
now) to a maximum safe speed on dry pavement. on the route with 11 curves or curve combinations compos-
ing the DPM observation sequences. The terrain is generally
gently rolling with a few steeper sections. There were no long
FIELD STUDY OF DRIVER BEHAVIOR grades. The environment ranged from a low-density suburban
USING DRIVER PERFORMANCE area with houses set back 75 feet or more from the traveled
MONITORING TECHNIQUE way to wooded and agricultural areas with farms and widely
scattered houses. The aerials of the DPM curve sequences
The project also included a driver observation study
also provide detail of the environment adjacent to the route.
using driver performance monitoring (DPM) techniques in
The prevailing traffic volumes were typical of rural roads,
which a sample of randomly selected drivers were observed
generally on the order of 1 to 2,000 vehicles per day. The
as they traversed an approximately 25-mile predetermined
state-numbered and -maintained road tended to be in better
route negotiating 43 curves. As part of the DPM data collec-
condition and to have better and slightly wider shoulders than
tion, detailed observations were made by trained observers at
did the county roads. Regulatory speed limits ranged from 45
11 curve sequences (some sequences contained more than
to 55 mph although some were not marked. For the latter, the
one curve) and included assessments of a driver's "search,
de facto limit in Michigan in rural areas is 55 mph. The
speed, and direction control" performance as they negoti-
curves themselves varied from very gentle with excellent vis-
ated each sequence. The vehicle's speedometer readings at
ibility, requiring no speed reduction or advisory speed signs,
various points were also recorded as were comments on
to a sharply descending right-angle turn, just after the crest
driving behavior.
of a hill, which had a 20 mph advisory speed plate. There was
As applied in this project, DPM is an observation tech-
one 2-mile section of gravel road and another section that
nique in which one or more trained observers ride with a sub-
was a quarter-mile to one-half-mile long. Most paved sections
ject driver (in the subject's vehicle) over a predetermined
had marked center- and edgelines.
route and make observations regarding driving behavior at
specific locations. The behavior of each driver is compared
with what is expected at each location. That expectation is DPM Subjects
established by observing other drivers at the same locations
on the same route prior to the data collection runs. While The 39 DPM subjects were randomly selected from online
DPM is a qualitative approach to evaluating driver behavior, and printed telephone books. The subject pool was con-
it is made as quantitative as possible by scoring each subject strained by telephone exchange so that they would be from the
as exhibiting satisfactory or unsatisfactory behavior on each area and, hence, more likely to participate. However, there
of three dimensions: visual search, speed, and direction con- was no attempt to randomize by age or to adhere to a rigor-
trol. An example of satisfactory behavior would be gradually ous statistical sampling regime as the purpose was not to
slowing down for a curve versus an abrupt speed change achieve statistically significant results. The 39 subjects were
right at the point of curvature. The observer(s) also makes split between day (28) and night (11) conditions. In addition
comments regarding any other occurrences such as a vehicle to the randomly identified subjects, there were four Michigan
pulling out of a driveway on the approach to a curve or other State Police (MSP) officers who drove the route and were
actions that might affect driver performance. The person in observed as if they were regular subjects. The MSP officers
the front seat is the principal observer making the judgments were instructed to drive at the edge of what they perceived as
on behavior while the back-seat observer makes ancillary the "safe envelope" so that an approximation of the maxi-
comments and observations such as speedometer readings at mum safe speed could be obtained. In spite of this direction,
various points along the route. DPM has been used in several one of the officers drove at the posted regulatory and advi-
contexts including as a driver-training tool for commercial sory speeds although he indicated that the "safe" speed was
truck drivers, as a diagnostic tool to evaluate whether recov- higher than those at which he drove--the data for this person
ering stroke victims are safe to resume driving, and as a were eliminated from all analyses.
research tool in an earlier NCHRP project on older drivers
(NCHRP Project 3-44, "Improved Traffic Control Device
Design and Placement to Aid the Older Driver"). DPM Results
As noted above, there are a number of quantifiable driver
General DPM Route Description behaviors derived from the observers' comments and obser-
vations. For example, each driver either performed satisfac-
The DPM route used in this project was an approximately torily or not on each of the standard DPM measurements of
25-mile loop in a primarily rural area southeast of Lansing, search, speed, and direction control through any given curve
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sequence. Note that a sequence might consist of as many as identified or they could be ordered by the frequency of a spe-
three or four sections. Likewise, observer summaries con- cific type of error. Likewise, the sequences could be ordered
tained indications of the type of errors, if any, that each driver by approach speed or speed in the curve. The summaries of
made as he or she traversed the sequence, although these were the subject performances can also be compared with the per-
not necessarily limited to the standard DPM measurements. It formance of the MSP officers or with other expectations of
is important to note that the words "error" and "problem," as performance. An example of the former is a comparison of the
used herein, refer to a deviation from expected driver behav- difference between the subjects' average speed for a specific
ior. Declaration of an "error" or a driver having a "problem" curve in a DPM sequence with the average (n = 3) from the
does not necessarily imply that a patently unsafe maneuver MSP officers. An example of the latter is comparison between
has occurred. Errors included not looking in the appropriate average subject speed and the posted regulatory and advisory
direction for an oncoming or intersecting vehicle, not slowing speeds.
down appropriately, an abrupt and/or late speed reduction, en- Another basis for comparison of driver performance is a
croaching on or crossing the center- or edgelines or both in the hierarchical ranking of "curve difficulty" or complexity based
case of a multiple-curve sequence, and handling difficulties on the TCDs currently deployed. This hierarchy was devel-
such as abrupt or potentially dangerous lane changing or oped by project personnel and is shown in Table 1. Each curve
encroachment. While not all portions of the route were marked on the DPM route can be assigned a number based on this
with edge- or centerlines, a centerline crossing was nonethe- hierarchy. Note that as used here, the hierarchy is based on
less noted when the driver crossed the approximate center of the TCDs currently deployed at any given curve and not, for
the roadway. The DPM performance assessment and other example, on any measure of the curve itself such as degree
comments were quantified and displayed in a spreadsheet. In of curvature or radius.
addition, the speeds collected by the back-seat observers After all the DPM observations had been made, short
were used to construct an approximate speed profile for the subject-based reports were written and comments were aggre-
entire route. Finally, the performances of all drivers on each gated for each sequence. Discussion was organized on the
DPM sequence were aggregated to provide overall "scores" basis of the hierarchy of curve-related TCDs deployed--
for the sequences. generally from the simplest curve to the most complex,
Given this performance-based information, the sequences based on existing signing. The hierarchy is subjective in
could be ordered in any one of several ways. For example, the nature but does provide a basis for ordering the discussion.
sequences with the highest numbers of driver errors could be The hierarchy is not without problems. For example, a curve
TABLE 1 Curve hierarchy based on TCDs deployed
Curve Ranking Signing Conditions
0 no curve
1 curve present, no sign
2 curve w/curve sign only
3 curve w/turn sign only
4 curve w/reverse turn sign only
5 curve sign + large arrow
6 turn sign + large arrow
7 reverse curve sign + large arrow
8 curve sign w/speed advisory
9 turn sign w/speed advisory
10 reverse curve sign w/speed advisory
11 curve sign + chevrons
12 turn sign + chevrons
13 reverse curve sign + chevrons
14 curve sign w/speed advisory + large arrow
15 turn sign w/speed advisory + large arrow
16 reverse curve sign w/speed advisory + large arrow
17 curve sign w/speed advisory + chevrons
18 turn sign w/speed advisory + chevrons
19 reverse curve sign w/speed advisory + chevrons
20 curve sign + other combination
21 turn sign + other combination
22 reverse curve sign + other combination
