Steer Threat Number (STN)¶

Description¶

Similar to the BTN, for two actors at time \(t\), the STN [Jansson2005] [Eidehall2011] is defined as the required lateral acceleration divided by the lateral acceleration that is at most available to \(A_1\) in that direction in that scene, i.e.

\[\mathit{STN}(A_1,A_2,t) = \frac{{a}_{\mathit{lat,req}}(A_1,A_2,t)}{a_{1,\mathit{lat,min}}(t)}.\]

By definition, an STN \(\ge 1\) indicates that a lateral maneuver performed by the actor cannot avoid an impeding accident.

Properties¶

Run-time capability¶

Yes

Target values¶

\(\ge 1\) (point of no return)

Subject type¶

Road vehicles (automated and human)

Scenario type¶

Same as \({a}_{\mathit{lat,req}}\)

Inputs¶

\({a}_{\mathit{lat,req}}\), \(a_{\mathit{lat,min}}\)

Output scale¶

\((-\infty,\infty)\), number, ratio scale

Reliability¶

High, under the assumption that the non-collision condition can be reliably predicted

Validity¶

Strongly depends on \({a}_{\mathit{lat,req}}\) and \(a_{\text{lat,min}}\) estimate; suited for inter-vehicle comparisons; no empirical analysis available

Sensitivity¶

High for non-humans, as steering is often optimal, but seldomly executed by humans [Adams1994]; strongly depends on \({a}_{\mathit{lat,req}}\) and direction of \(a_{\mathit{lat,min}}\) estimation

Specificity¶

High, but strongly depends on \({a}_{\mathit{lat,req}}\) and direction of \(a_{\mathit{lat,min}}\) estimation