Yiik-Diew Wong

ARE ACCIDENTS POISSON DISTRIBUTED? A STATISTICAL TEST

The common and convenient assumption in accident count analysis, that accidents are Poisson-distributed, is reexamined. Two statistical tests, for evaluating the assumption are described and compared. It is shown that a test based upon a combinatorial analysis is much more accurate than the alternative chi-square test when accident counts are expected to be small. The more accurate test is used to reinterpret data on accident count variability, the results indicating that the Poisson distribution is appropriate for the analysis of accidents at individual sites.

Micro-simulation of vehicle conflicts involving right-turn vehicles at signalized intersections based on cellular automata.

At intersection, vehicles coming from different directions conflict with each other. Improper geometric design and signal settings at signalized intersection will increase occurrence of conflicts between road users and results in a reduction of the safety level. This study established a cellular automata (CA) model to simulate vehicular interactions involving right-turn vehicles (as similar to left-turn vehicles in US). Through various simulation scenarios for four case cross-intersections, the relationships between conflict occurrences involving right-turn vehicles with traffic volume and right-turn movement control strategies are analyzed. Impacts of traffic volume, permissive right-turn compared to red-amber-green (RAG) arrow, shared straight-through and right-turn lane as well as signal setting are estimated from simulation results. The simulation model is found to be able to provide reasonable assessment of conflicts through comparison of existed simulation approach and observed accidents. Through the proposed approach, prediction models for occurrences and severity of vehicle conflicts can be developed for various geometric layouts and traffic control strategies.

Key risk indicators for accident assessment conditioned on pre-crash vehicle trajectory

Accident events are generally unexpected and occur rarely. Pre-accident risk assessment by surrogate indicators is an effective way to identify risk levels and thus boost accident prediction. Herein, the concept of Key Risk Indicator (KRI) is proposed, which assesses risk exposures using hybrid indicators. Seven metrics are shortlisted as the basic indicators in KRI, with evaluation in terms of risk behaviour, risk avoidance, and risk margin. A typical real-world chain-collision accident and its antecedent (pre-crash) road traffic movements are retrieved from surveillance video footage, and a grid remapping method is proposed for data extraction and coordinates transformation. To investigate the feasibility of each indicator in risk assessment, a temporal-spatial case-control is designed. By comparison, Time Integrated Time-to-collision (TIT) performs better in identifying pre-accident risk conditions; while Crash Potential Index (CPI) is helpful in further picking out the severest ones (the near-accident). Based on TIT and CPI, the expressions of KRIs are developed, which enable us to evaluate risk severity with three levels, as well as the likelihood. KRI-based risk assessment also reveals predictive insights about a potential accident, including at-risk vehicles, locations and time. Furthermore, straightforward thresholds are defined flexibly in KRIs, since the impact of different threshold values is found not to be very critical. For better validation, another independent real-world accident sample is examined, and the two results are in close agreement. Hierarchical indicators such as KRIs offer new insights about pre-accident risk exposures, which is helpful for accident assessment and prediction.

Multi-objective optimal speed for expressways

Relationships for speed–flow, speed–fuel consumption (speed–carbon emission rates) and speed–accident occurrences for highway (expressway) driving conditions are reviewed. The optimal speeds for maximising traffic flow capacity on highly utilised expressways in Singapore lies in the range of 45–65 km/h, a speed band that the Land Transport Authority (Singapore) uses in its Electronic Road Pricing (ERP). On the other hand, the optimal speed range lies between 50 and 75 km/h based on efficient fuel consumption. As for carbon emission rates of the vehicular flow, the optimal speed range is 50–60 km/h. In terms of accident occurrences, the general norm is that low travel speeds as well as low speed variability are desirable. Experimental runs have shown that a relatively low speed band is preferred for greater fuel efficiency. Setting operating speeds that straddle the optimum regimes of capacity, safety and environmental sustainability suggests a range of 50–60 km/h.

Pedestrian archeology in Singapore: experiences from the past

Walking is an important part of the transportation system, as everyone walks. Providing an adequate infrastructure network promotes walking. Additional walking facilities encourage longer walks, especially in the tropical climate of Singapore. This paper provides a good documentation of the historical infrastructural developments to enhance the walking experience in Singapore, from post World War II to present. Lessons learnt from these past experiences are highlighted.