The hard-braking event data used in this study were made commercially available by data providers that worked directly with original equipment manufacturers (OEMs). The enhanced probe data from these connected passenger vehicles included an anonymized unique identifier with timestamp, geolocation, speed, heading and hard-braking/acceleration as attributes. The provider of this data defined hard-braking events as any vehicle decelerations with a magnitude greater than 8.76 ft/s2 (0.272 g).
In July 2019, there were over 1.5 million events in Greater Indianapolis (Fig. 1b), almost 16,000 events along the study corridor (Fig. 1c), and about 10,000 events at the corridor’s eight intersections. Every weekday, those intersections experienced an average of 321 events, and every hour, they experienced an average of 14 events. The penetration level of this data is estimated to be around 2%.
The hard-braking events analyzed in this paper were sorted by intersection, distance from stop bar, and speed at which the vehicle was traveling when the hard-braking event occurred. In this study, the analysis was limited to through movements. A geofence region was drawn along the through lanes for each approach. This upstream region began parallel to the opposing direction’s stop bar and ended 1320 ft, a quarter mile, upstream. Once the geofenced region was defined, the hard-braking events that occurred within those regions were selected, and the GPS locations of each hard-braking event were compared to the location of the stop bar to calculate the distance from stop bar. Figure 2a shows the hard-braking events for an area along the study corridor. Figure 2b shows the upstream geofence regions and the geofenced hard-braking events color coded by speed. The 400 ft boundary, relative to the stop bar, roughly corresponds to the location of the dilemma zone detectors at this intersection (Gazis et al. 1960; Parsonson 1978; Zegeer and Deen 1978).
Analysis: Hard-Braking Events by Approach
The hard-braking events are classified by their distance from the stop bar to study the impact of dilemma zone (Gazis et al. 1960; Parsonson 1978; Zegeer and Deen 1978) and queuing. Type II dilemma zone has been defined in previous literature as the road segment where there is a 10–90% probability of a vehicle stopping at the beginning of the yellow light (Parsonson 1978). The occurrence of hard-braking events less than 400 ft (location of advance detector upstream of stop bar at 55 MPH speed limit zone) at lower speeds are possibly due to vehicles stopping for the red light, whereas such occurrences at higher speeds could be due to dilemma zone issues. Hard-braking events occurring at distances greater than 400 ft from the stop bar are potentially due to long queues during oversaturated conditions.
Figure 3 shows the number of weekday hard-braking events occurring at each intersection, stacked by distance from the stop bar, aggregated over the month of July 2019. For both NB and SB approaches, the majority of the hard-braking events occur within 400 ft of the stop bar. However, there are a few intersections (#8 Smith Valley Rd., in NB and #4, Southport Rd., and #5, Wicker Rd. in SB) where more than 40% of hard-braking events occurred more than 400 ft from the stop bar.
To understand the temporal nature of the hard-braking events and their distances from the stop bar, a heatmap was generated. Figure 4 illustrates a heatmap of the number of hard-braking events, during weekdays in July 2019, on the NB approach over a 24-h period (30-min bins) across two distance categories—less than 400 ft and greater than 400 ft. For the less than 400 ft category, the majority of hard-braking events occur during the AM, MD and PM plans (Fig. 4a), with no clear pattern or trend. For the 400–1320 ft range (Fig. 4b), there are generally fewer hard-braking events, except for perhaps intersection 8 during the PM plan.
Figure 5 shows a heatmap similar to Fig. 4, for the SB approach. Hard-braking events within 400 ft of the intersection (Fig. 5a) are generally higher for the PM plan, especially at intersection 8, Smith Valley Rd. Figure 5b, which comprises of events occurring beyond 400 ft, shows a different pattern than the northbound approaches. Intersection 4, Southport Rd., and intersection 5, Wicker Rd., experience a large number of hard-braking events during the PM plan. This could be indicative of hard-braking events that occur at the back of long queues during the PM peak period.
Analysis: Hard-Braking Patterns by Intersection
To further investigate the pattern of hard-braking events, a histogram of the events stacked by speeds are plotted for different time of day plans over their distance from the stop bar. Figures 6 and 7 present the two such patterns, in regard to intersections along the SR-37 corridor, for weekdays between 5:00 AM and 10:00 PM in July 2019.
Figure 6 shows the hard-braking events at the southbound approach of intersection 4, Southport Rd. During the PM time plan (Fig. 6b), hard-braking events are occurring consistently for the entirety of the quarter-mile from the stop bar, with very few of those hard-braking events occurring at speeds over 45 mph. The aerial image in Fig. 6a shows that there are no driveways or bus stops in the region that could be contributing to these hard-braking events.
Figure 7 shows the hard-braking events at the southbound approach of intersection 8, Smith Valley Rd. The PM plan, (Fig. 7b), stands out as having numerous hard-braking events within the 0–400 ft region. In some of the speed bins around 250 ft upstream of the intersection, over 60% of those hard-braking events occur at speeds above 45 mph. Dilemma zone protection is often difficult on coordinated movements as more phases compete for green time and coordinated phases are forced off.