Time To Zebra (TTZ)¶

Description¶

Defined by Várhelyi et al., TTZ measures the time until an actor \(A_1\) reaches a zebra crossing \(\mathit{CA}\) [Varhelyi1998], hence

\[\mathit{TTZ}(A_1,\mathit{CA},t) = \min \; (\{ \tilde{t} \ge 0 \,\mid\, d(p_1(t+\tilde{t}), p_{\mathit{CA}}(t+\tilde{t})) = 0 \} \cup\ \{ \infty \}).\]

Note that this concept can be further generalized to a Time To Object (TTO) metric for arbitrary moving or non-moving objects and conflict areas. For moving objects, this generalization coincides with the TTC.

Properties¶

Run-time capability¶

Yes

Target values¶

\([2,4]\) s [Varhelyi1998] (critical interval at time of arrival of VRU)

Subject type¶

One road vehicle

Scenario type¶

Any scenario where a road vehicle is approaching a pedestrian crossing

Inputs¶

State of the approaching road vehicle, position of pedestrian crossing

Output scale¶

\([0,\infty]`\), time (s), ratio scale

Reliability¶

Low, as changes in criticality are likely not reflected in TTZ, e.g. changes in walking directions of pedestrians

Validity¶

Low, as only the pedestrian crossing is regarded to be safety-relevant, and impact of other road users is not considered; no empirical analysis available

Sensitivity¶

Low, as VRUs can potentially cross in front of a pedestrian crossing, leading to critical scenarios with high TTZ values

Specificity¶

Low, if no VRU is present and the vehicle is close to the crossing, the scenario is uncritical, but TTZ approaches 0