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10 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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11 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