# Selecting Non-Line of Sight Critical Scenarios for Connected Autonomous Vehicle Testing
## 2 Related Work
### Metrics  
To evaluate the safety of a concrete scenario, Key Performance Indicators (KPIs) are required. Since accidents are rare, criticality metrics such as KPIs are advantageous [52]. The most well-known criticality measure is time-to-collision (TTC) [53]. The goal is to detect scenarios which jeopardise vehicle safety by using criticality measurements. Some of the metrics used in previous works are summarised in Table 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Indicator** & **Description** & **Collision Type** & **Reference** \\ \hline Time-to-collision (TTC) or & The time it would take for the & Rear-end collision, & & \\ Time-to-collision (TTC) or & vehicles to collide if they & weaving/turning, colliding with & & [54, 55] \\ Time-measured-to-collision (TMTC) & continued on their current paths & objects/parked vehicles, crossing & & \\  & at their current speeds. & and colliding with a pedestrian. & & \\ \hline Time exposed time-to-collision (TET) & Sum of all times (during the time & As a result of a rear-end collision, & & \\ Time exposed time-to-collision (TET) & period under consideration) when a driver approaches a front & turning/weaving, hitting & & \\  & vehicle with a TTC value less than & objects/parked vehicles, crossing, & & \\  & vchicle with a TTC value less than & hitting a pedestrian and & & \\  & TTC. & vehicle–vehicle collision. & & \\ \hline Time integrated time-to-collision & The integral when TTC is below & As a result of a rear-end collision, & & \\ (TIT) & the threshold. & turning/weaving, hitting & & \\  & & objects/parked vehicles, crossing, & & \\  & the threshold. & hitting a pedestrian and & & \\  & & vehicle–vehicle collision. & & \\ \hline  & Models that were modified to & As a result of a rear-end collision, & & \\  & account for all possible & turning/weaving, hitting & & \\ Modified time-to-collision (MTTC) & longitudinal conflict scenarios & objects/parked vehicles, crossing, & & [57, 58] \\  & caused by acceleration or & hitting a pedestrian and & & \\  & deceleration discrepancies. & vehicle–vehicle collision. & & \\ \hline  & & As a result of a rear-end collision, & & \\  & The effect of collision speed on & turning/weaving, hitting & & \\  & the kinetic energy involved. & objects/parked vehicles, crossing, & & \\  & hitting a pedestrian and & & \\  & vehicle–vehicle collision. & & \\ \hline Headway (H) & The time it takes for the front of & As a result of a rear-end collision, & & \\  & the leading vehicle to pass a point & turning/weaving, hitting & & \\  & on the road and for the front of & objects/parked vehicles, crossing, & & \\  & the following vehicle to pass the & hitting a pedestrian and & & \\  & same point. & vehicle–vehicle collision. & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Review of Safety Indicators.  
As mentioned earlier, TTC is a well-known representative temporal metric and it is presented in the works of [PERSON] et al. [65]. The TTC measure, while simple to calculate and interpret, is unable to account for the participants' acceleration and deceleration behaviour because it is a stationary metric. As a result, combining different metrics from different basic methods for a comprehensive assessment is required. This paper will discuss combining different metrics in order to determine NLOS safety-critical scenarios.