A framework for reliability-based weather-responsive speed management for rural highways located in extremely cold regions

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Rillagodage, Navoda Yasanthi
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Weather-responsive variable speed limit (WRVSL) systems attempt to regulate drivers’ speed choice in adverse road-weather conditions (RWCs) by executing a set of differentiated speed limits implemented under different RWCs. This thesis attempts to propose an effective approach to set WRVSLs in cold region rural highways based on the reliability of a WRVSL — the probability of a WRVSL being safe and complied by drivers. This thesis addresses four research questions: (i) What is the impact of different RWCs on drivers’ speed choice? (ii) What is the most appropriate approach to evaluate drivers’ speed behaviour in adverse RWCs? (iii) Are there any specific driving conditions which intensify safety risks when indicated by speed and speed variability? (iv) What is an effective approach to regulate speed in adverse RWCs to mitigate safety risks induced by such RWCs? The first research question is addressed by proposing an approach to model speed distributions for different combinations of traffic and road-weather conditions using the central limit theorem (CLT). The second research question is addressed by comparing the CLT-based speed distribution modelling approach proposed in this thesis with a regression modelling based approach. To address the third research question, a holistic crash indicative measure is proposed based on both within- and across-lane crash risks (estimated based on speed variability) for different driving conditions such as prevailing RWCs, vehicle type, travel lane, and truck payload condition. The fourth research question is addressed by proposing an effective approach to set WRVSLs based on the reliability theory. Overall, this thesis demonstrates that extreme RWCs such as snow, and icy pavements notably affect drivers’ speed choice, resulting increased speed variability, thus intensified crash risks in such RWCs. The results of this research also revealed that tractor-trailer combinations are highly vulnerable when travelling in extreme RWCs. Practical applications of the proposed research methods include (i) accurately understanding driver’s speed choice patterns in different RWCs, (ii) developing RWC-specific speed distributions that could be used as inputs in microsimulation studies, (iii) identifying RWCs with intensified crash potential based on speed and speed variability, and (iv) effectively regulating drivers’ speed choice in extreme RWCs.
Adverse road-weather conditions, Speed distributions, Variable speed limit, Speed behaviour, Traffic safety