What are the advantages and challenges of using surrogate safety measures in traffic engineering?

1 What are SSMs?

SSMs are indicators or proxies of traffic safety that are derived from non-crash data, such as traffic speed, volume, flow, density, headway, gap, time-to-collision, post-encroachment time, and so on. SSMs can capture the frequency and severity of traffic conflicts, near-misses, and critical situations that may lead to crashes, but do not necessarily result in them. SSMs can also reflect the exposure and vulnerability of different road users, such as pedestrians, cyclists, and motorists, to traffic hazards. SSMs can be obtained from various sources, such as field observations, video recordings, sensors, radar, GPS, or simulation models.

2 Why use SSMs?

SSMs have several advantages over crash data for traffic safety analysis. First, SSMs are more abundant and available than crash data, which means they can provide more information and insights into the traffic safety performance of a road network or a specific location. Second, SSMs are more proactive and preventive than crash data, which means they can identify and address potential safety problems before they result in crashes. Third, SSMs are more sensitive and responsive than crash data, which means they can capture the effects of various factors and interventions on traffic safety, such as weather, traffic demand, road design, traffic control, enforcement, education, or engineering measures.

3 How to use SSMs?

SSMs can be used for a variety of traffic engineering purposes, such as safety diagnosis, evaluation, prediction, and optimization. For example, SSMs can help to identify locations with high levels of traffic conflicts or near-misses and to rank them according to their safety risk. Additionally, SSMs can be used to compare the safety performance of different alternatives or scenarios and to estimate the expected number or rate of crashes or injuries using statistical or mathematical models. Finally, SSMs can also be used to find optimal solutions or strategies to improve traffic safety, such as selecting the best road design, traffic control, or engineering measures.

4 What are the challenges of using SSMs?

Despite their advantages, SSMs (Safety Score Metrics) also have some challenges and limitations when it comes to traffic engineering. Validity and reliability must be established to ensure the SSMs reflect the actual or perceived safety risk or performance of a road network or location. Data quality and availability are important too, as the non-crash data used for deriving SSMs must be of high quality and sufficient coverage. Additionally, interpretation and communication are key for SSMs to be understood by stakeholders such as decision-makers and the public. Furthermore, SSMs need to be coordinated with other safety indicators or measures, like crash data or safety audits, to create a comprehensive view of traffic safety.