Example animation explaining the Surrogate Safety Measure TTC (Time To Collision).
Traditionally, accident data has primarily been used to analyse the road safety of a traffic facility. However, this is a reactive approach, which means that accident occurrences must first be observed over many years before measures can be taken to improve road safety. In addition, the accident statistics show a certain number of unreported cases and there is only limited information on the sequence of events that led to the accident.
Assuming that accidents are only the tip of the iceberg, traffic flow can also be observed and analysed in terms of near misses in a complementary or supplementary manner. In a simplified way, the traffic flow can be divided into the following hierarchy of severity: Undisturbed passages - Interactions - Near misses - Accidents. The near misses (similar to accidents) can be divided into further severity levels with regard to the proximity to a collision, but also with regard to the severity of the resulting collision (similar to accident severity).
A number of Surrogate Measures of Safety have been developed as objective indicators to classify an interaction as a near miss and to assess its severity (primarily in terms of proximity to a collision). One commonly used indicator is Time To Collision (TTC).
The TTC describes the remaining time to a collision between two road users at a certain point in time of their interaction and under the assumption that neither of the parties involved will take an evasive action. The lower the TTC, the more severe the conflict between road users. In case of a collision the TTC equals zero.
A TTC can only be calculated if the two interacting road users are on a collision course. If the collision course is left due to an evasive action, the TTC for the time from the start of the evasive action is calculated on the basis of the hypotethic trajectories without the evasive action. Such hypotethic trajectories can be approximated by various methods of trajectory prediction. The simplest (but in some cases also the most unrealistic) method for trajectory prediction is to assume continued motion with constant velocity vectors.
The TTC is calculated for each moment of interaction. This results in a TTC curve. Interactions and also periods of interactions are often categorised according to whether the TTC is below a threshold value. The Minimum TTC, the Time Exposed TTC and the Time Integrated TTC are used as aggregated indicators. In order to enable comparisons, e.g. with other traffic facilities, these parameters are related to suitable exposure units, such as traffic volume or the total number of interactions in the observation period.
In addition to road safety analysis, TTC is also used for active accident avoidance, especially for decision-making in algorithms of autonomous vehicles.
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