A road authority wanted to understand why two intersections had high collision rates.
Intersection 1
At the first T-intersection, collisions were occurring for vehicles travelling straight and those turning right onto the main carriageway. Path Analysis revealed drastic drops in average speeds along the carriageway: northbound cars approaching the intersection slowed from 60km/h to 20km/h (37mph to 12mph), and southbound cars from 59km/h to 28km/h (36mph to 17mph).
Similar drops in speed continued to occur further along the road at other intersection points. Most vehicles were within the 80km/h (50mph) speed limit, but had a great variability in speeds. Approximately 5% of drivers travelled above the limit, and 40% travelled 10-20km/h (6-12mph) below the limit. This is likely due to vehicles joining the carriageway from residential offshoots and transitioning from a 50km/h (31mph) limit, possibly contributing to collisions.
Chainage graph shows big spikes in average speeds along the main carriageway.
Speed bins show proportionality of traffic travelling at different speeds, but still mostly within the 80km/h limit.
Intersection 2
G-forces at the second T-intersection under investigation showed a high concentration of g-forces above 0.3G along the main carriageway, particularly for vehicles travelling westbound. These suggested vehicles were navigating conflict points when turning or merging.
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G-force map layer shows a high concentration of g-forces above 0.3G.
Interestingly, Path Analysis showed that once cars on the main carriageway passed the intersection, average speeds steadily decreased from ~55km/h to 27km/h. There were also high acceleration events above 0.2G for the 85th percentile turning into the residential street from the main carriageway.
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Chainage graph shows a steady decrease in speed after the intersection
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A review of the site on Google Maps showed a nearby petrol station and shops, with vehicles parking on the left-most lane of the carriageway. Also, for vehicles turning right from the residential street onto the main carriageway, only a small space in the middle of the intersection was provided. This combination of conflict points in the 70km/h speed zone was likely contributing to collisions.
This helped the authority to identify risk factors behind reported accidents, assess if enforcement was required and provide insights to road designers.