Road safety analyses have traditionally used crash data as direct measure of road traffic safety and counteractions (Tarko, Davis, Saunier, Sayed, & Washington, 2009). The use of crash statistics only does not create an efficient ground in identifying and analysing road safety problem at particular location and then suggest countermeasures (Van der Horst, 1984). Traffic safety measures that rely on the traditional accident data have been frequently questioned for the reason that of shortcomings associated with crash data (Yang, 2012).
The shortcoming related with crash data may include that accidents are rare and highly random events that usually require extended observation time in which many extraneous factors may happen; a significant number of crashes have to be recorded before analysis; and quality of crash data (e.g., incomplete report) (Ismail, Sayed, Saunier, & Lim, 2009). Due to such limitations, researchers have developed an interest to use complementary measure to investigate road safety. Such complementarity studies are surrogate safety measures.
The word “surrogate” indicates an alternative to measure for safety analysis that do not rely on accident data (A Laureshyn, Varhelyi, & Svensson, 2015). The concept of surrogate safety measures indicates measures other than actual crash data, and they would be important to study the safety of traffic facilities without waiting to get significant number of crash record, and experiment a roadway designs prior they are actually employed (Gettman, Pu, Sayed, Shelby, & Siemens, 2008; Ismail, Sayed, Saunier, & Lim, 2009).
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General Motors developed traffic conflict technique to investigate whether its vehicles were driven differently from others; however, the technique was soon applied to evaluate accident potential without depending on accident data (Gettman & Head, 2003). Historically, using surrogate measures for safety analyses became popular in 1970 – 80s (C Hydén, 1996), especially after the first attempt of safety investigation made through observation of traffic conflicts in the 1950 – 60s (Perkins & Harris, 1967 in A Laureshyn et al., 2015). Although surrogate measures were less used after the beginning of 1990s, due to technological advancement there has been renewal in users’ interest to employ surrogate measures in safety analysis (Aliaksei Laureshyn, 2010).
Surrogate Safety measures can be applied in several aspects of traffic safety analyses including travel time, queue lengths, red-light violations, left turns, speed distribution and so forth (Gettman et al., 2008). More specifically, they are dominantly applied to traffic conflicts that are observable situations, in which road user come each other in distance and time to such degree there would be risk of collision if the movement among them remain unchanged (Amundsen & Hyden, 1977). A key component of surrogate safety measure is the traffic conflict technique (TCT), which involves the analysis of observed traffic conflicts (Ismail, Sayed, & Saunier, 2011).
Surrogate safety measures possess important advantages over road traffic collisions analysis, for the reason that traffic conflicts are more clearly observed, and are much less costly than road collision (Ismail et al., 2011), time effective in data collection process, available information than accidents, and proactive measure (Polders & Brijs, 2018). Although conflict studies have advantages, they involve some drawbacks, for example, costs associated with data collection devices and processing software; and inconsistent relationship between conflicts and accident (Ismail et al., 2011).
