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