Surrogate safety indicator for unsignalised pedestrian crossings

Piotr Olszewski , Paweł Dąbkowski , Piotr Szagała , Witold Czajewski , Ilona Buttler

Abstract

Problem:Although the road safety situation in Poland is generally improving, the numberof accidents at pedestrian crossings has not decreased in the last four years. This paper pre-sents the results of the MOBIS research project, the aim of which was to develop surrogatesafety indicators, based on detection of pedestrian-vehicle conflicts using video analysis.Method:Pedestrian and vehicle traffic was filmed at two unsignalised pedestrian zebracrossings in Warsaw and Wrocław for over 40 days. Motion trajectories of vehicles andpedestrians were determined based on video processing. Several variables describingpedestrian-vehicle interactions were calculated, such as speed, post-encroachment time,distance between the participants, decelerations, etc. Classification of encounters wasbased on interactions of pedestrians and vehicles i.e. drivers yielding to pedestrians,vehicles passing just in front of, or behind pedestrians.Results and discussion:Criteria for identification of dangerous encounters were selectedwith the assumption that it should be possible to automate the assessment process. Theselected variables were: pedestrian-vehicle passing distance and the vehicle speed at thatmoment. Other criteria were used in cases of abrupt braking – deceleration exceeding4 m/s2and vehicle speed. A Dangerous Encounter Index is proposed as a surrogate safetyindicator for pedestrian crossings. It relates the occurrence of dangerous events toexposure, defined as the number of pedestrian-vehicle encounters.Practical applications:The proposed index shows that crossing two lanes involves more riskthan crossing one lane, given similar traffic flow. Some improvement of safety at both typesof crossing was observed after active signage involving blinking lights had been introduced.The proposed method is a step towards automation of safety assessmen