Lateral control is safety-relevant. Lane departures are the first critical event in single-vehicle road departure crashes, lane change crashes, and opposite direction crashes. Jointly, these crash types represent a substantial portion of the crash problem. The intersection of these types of crashes with driver distraction causal factors provides the motivation for measuring and analyzing lateral control performance.
Many different measures of lanekeeping can be defined. In the DWM project, two measures have been selected for in-depth evaluation. The rationale behind these selections is provided below.
Standard Deviation of Lane Position (SDLP) is defined as the square root of the average
squared deviation in lane position about the mean lane position observed during a task. It is measured in feet, and was obtained with an Assistware Technology, Inc. lane tracker. SDLP was
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calculated only if valid lane tracker data were available for 85 percent or more of a task trial duration. For each participant, each task trial generated an SDLP value. For each participant, the SDLPs were averaged over his or her replications of a given task. This created a single SDLP value for that participant performing that task. If only a single trial was available for a participant, then its SDLP value was used for that task from that participant. The median of all such SDLPs for a given task was used as a summary statistic for that task. The median was chosen because it is more resistant than the mean to outliers or extreme values in the data. Its use avoids the need for data truncation or data transforms to approach normality. The median can provide a robust “typical” value of task effects.
Percent Lane Exceedance(Cross) Trials is the percentage of participants who had one or more
lane exceedances during one or more trials for a given task. A lane exceedance (cross) event was defined to have occurred if the leading edge of the subject vehicle crossed the adjacent lane line's outer edge. For each participant, each task trial generated a Lane Exceedance count. These counts were converted to a binary score, one (1) if there were one or more lane exceeds, zero (0) otherwise. For each participant, these binary scores were averaged over his or her replications for a given task. This created a single Lanex (Cross) Trials value for that participant performing that task. If only a single trial was available for a participant, then its Lanex (Cross) Trials value was used for that task from that participant. The average of these values was calculated and multiplied by 100 to create Lanex (Cross) Trials percentages. Lane exceedances were manually verified by staff through review of track pavement video to confirm lane tracker output. This manual method did not allow for measuring the lateral extent of a lane exceedance. The lane tracker did not always provide reliable data on the lateral extent of a lane exceedance if the lane tracker lost the lane line(s). Lateral extent of a lane exceedance was not assessed for this reason. Finally, the percentage of participant cross trials was used in lieu of lane exceedance counts because test participants did not all complete the same number of trials per task.
These two measures provide complementary data to distinguish task effects. Lane exceedances are discrete and infrequent events that can provide an indication of degraded lanekeeping. Lane exceedances are also of procedural interest. From a practical standpoint, lane exceedance event counts and durations out-of-lane can be captured through video and can be manually reduced and verified. Manual review of video, however, does not allow accurate assessment of the lateral extent or overridden area.
SDLP on the other hand, is a continuous, ever-present measure of lanekeeping. Normal probability theory suggests that larger SDLP implies an increased likelihood of departing the travel lane eventually (Allen, Parseghian, and Stein, 1996). While there may be no lane exceedances for a given trial, there is always lanekeeping to measure. However, SDLP requires a lane tracking system for data capture. Such systems, like eye trackers, are at least sometimes less than robust. They are also expensive.
Lateral control usually is more sensitive to task effects than longitudinal control because lateral position can change more quickly. It has proven useful in studies of driver distraction, drowsy driving, and intoxication. Police use lateral control as an indicator of impaired driving because, over years of real-world experience, it is relevant. On the other hand, measures based on steering inputs have not been used in this analysis because of the noisiness of steering data in the face of road and vehicle characteristics, and the more remote association to lanekeeping. However, measures based on steering inputs have been used successfully for over 30 years (e.g., McLean and Hoffman, 1975) and merit future evaluation.
3.5.1 Standard Deviation of Lane Position (SDLP)
Figure 3-40 presents the median SDLP for the 23 tasks evaluated on the test track. The range of median SDLP values was between 0.45 ft and 0.8 ft, within the normal range. The data are
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ordered in such a fashion that task duration may play a role in interpretation of the results. For example, Just Drive had greater median SDLP values than almost all other tasks except the two most visually demanding tasks performed on the track (Destination Entry and Route Tracing) and the Delta Flight Information mixed-mode task.
For reference, Figure 3-41 provides the median track task durations for all tasks. Auditory-vocal tasks (except Book-on-Tape Summarize) and Just Drive were of fixed durations (approximately 2 min). Visual-manual tasks were duration-intrinsic. That is, visual-manual tasks were not of fixed duration, they took as long as the test participant spent to complete them.
Task Effect on Track Median SDLP
0 0.2 0.4 0.6 0.8 1 HVAC RadioEasy BookOnTapeSumm RadioHard Cassette CDTrack7 VoiceDial MapEasy ReadHard ManualDial Coins ReadEasy RouteInstruct Sports MapHard BiographicQA TravelComp BookOnTapeListen RouteOrient JustDrive Delta RouteTracing DestEntry Tas k Median SDLP (ft)
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