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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-10-068
Date:November 2010 |
Crosswalk Marking Field Visibility Study
CHAPTER 5. DATA REDUCTION
PARTICIPANT DEMOGRAPHICSRESPONSE TIMEDETECTION DISTANCEAPPEARANCE AND PREFERENCE RATINGSPOSTDRIVE RANKINGS
PARTICIPANT DEMOGRAPHICSTable 7 lists the demographic information for the 78 participants. The original goal was to have 32 participants age 55 or older. That goal was exceeded, with 35 older participants in the study. The large number that selected retired for employment (33 percent) is a reflection of the emphasis on having half of the participants over 55 years of age. Table 7. Demographic information for 78 participants.
RESPONSE TIMEThe response lag times were determined for each subject. Two experimenters collected all the data, with one experimenter always in the SUV and the other experimenter always in the sedan. Initial effort determined the average response time by experimenter for all the participants. A review of the data revealed several outliers, such as when the response was more the 3 s. To eliminate these outliers, responses of more than 3 s or less than 0.4 s were removed from the data set. The long response times were deemed to be caused by some distraction on the part of the participant or the experimenter, which happened occasionally in the intake room. The very short response times were eliminated because they were cases in which the experimenter accidentally pressed the button before the participant spoke. In addition, data were eliminated if the response time was greater than two standard deviations of the subject's average. These steps removed 177 responses (about 10 percent). Table 8 lists the average response time by experimenter before and after removing data. In general, the response time was about 1 s for either experimenter. Table 8. Response time by experimenter.
A more detailed review of the response time data indicated that adjusting the detection distance should occur uniquely for each participant rather than using a per–experimenter average response time. Figure 38 shows the plot of the responses measured for each participant before eliminating the outliers. Figure 39 shows the plot of the responses measured after eliminating the outliers. As can be seen in the plots, some participants had average response times below 0.8 s while other participants' response times averaged above 1.2 s. Therefore, the average response time by participant rather than by experimenter was used to adjust the detection distance.
Figure 38. Graph. Measured response times by vehicle/experimenter and participant.
Figure 39. Graph. Response times by vehicle/experimenter and participant after removing outliers. The measured detection distance was adjusted using the average response time for the participant and the speed of the vehicle at the point when the participant said "crosswalk." The resulting adjustments ranged from 0 to 41 ft. Figure 40 illustrates the adjustments by participant number. The very low adjustments were for the crosswalks at stop-controlled intersections. To illustrate the type of adjustments used at the subject intersections, the minimum and maximum adjustments used for the higher speed F&B crosswalks are shown in figure 41 . As shown, the minimum adjustment for the F&B crosswalks was 15 ft and the maximum was 41 ft. For the nine crosswalks installed for this study, the adjustments ranged from 8 to 41 ft on detection distances that averaged 318 ft.
Figure 40. Graph. Response distance for all crosswalks.
Figure 41. Graph. Response distance for F&B crosswalks.
DETECTION DISTANCEThe Dewesoft software package synchronizes the GPS and video data stream records. The synchronized data were used to determine several items of interest such as the number of pedestrians and bicyclists in the participant's view and the velocity and GPS coordinates when he or she identified crosswalks. The response time determined for each participant was used along with the DAS data to obtain the detection distances. Data reduction included several steps. Dewesoft Exports The GPS data from the Dewesoft program were exported into spreadsheets. The time and GPS coordinates when the participant said "crosswalk" were identified within the data streams. The GPS location of each crosswalk was recorded before the study began. The detection distance was determined by subtracting this distance from the location marked by the experimenter in the vehicle. This calculated distance was then adjusted to account for the response time of the experimenter and participant. Average response time of the experimenter for that subject was multiplied by the velocity at the time of crosswalk identification to obtain the response distance. The response distance was added to the detection distance to obtain the adjusted detection distance. Pedestrian, Bicyclist, and Influencing Vehicular Traffic Presence The number of pedestrians and bicyclists in the participant's view when approaching a crosswalk was determined using the video data. The number of pedestrians was subdivided into whether the pedestrian was moving toward the crosswalk or away from the crosswalk and if the pedestrian was in between the crosswalk and the vehicle. The view for recording the number of pedestrians was subdivided into the following three areas:
During data reduction, the experimenter also judged whether surrounding vehicular traffic was affecting the participant's ability to see the crosswalk. Table 9 shows the basic format for the data reduction sheet. Table 9. Sample data reduction sheet for number of pedestrians.
APPEARANCE AND PREFERENCE RATINGSThe appearance rating given for each crosswalk site was recorded on the participant's data sheet. Participant comments were also recorded (when provided) to aid in understanding the reason for a participant's response.
POSTDRIVE RANKINGSThe order of the photographs as indicated in the postdrive ranking task was also recorded on the participant's data sheet. These rankings were transferred from the data sheets into spreadsheets to facilitate evaluations during data reduction. Because the number of participants within a group (e.g., day versus night or sedan versus SUV) was not exact, the frequencies were converted into proportions.
FHWA-HRT-10-068
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