Brake Threat Number (BTN)#

Description#

For actor \(A_1\), the BTN [Jansson2005] is defined as the required longitudinal acceleration imposed on actor \(A_1\) by actor \(A_2\) at time \(t\), divided by the longitudinal acceleration that is at most available to \(A_1\) in that scene, i.e.

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

By definition, a BTN \(\ge 1\) indicates that a braking maneuver performed by the actor cannot avoid an impeding accident under the assumed DMM. An extension of BTN to multiple actors is proposed by Eidehall [Eidehall2011].

A special case of the BTN is known as the Deceleration-based Surrogate Safety Measure (DSSM). Here, for car-following scenarios, a worst case assumption of maximum braking of the lead vehicle is combined with an acceleration-dependent estimation of the following driver’s time to perceive the threat and transition to emergency braking, thus incorporating human factors into the model [Tak2015].

Properties#

Run-time capability#

Yes

Target values#

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

Subject type#

Road vehicles (automated and human)

Scenario type#

Same as \(a_{\mathit{long,min}}\)

Inputs#

\(a_{\mathit{long,req}}\), \(a_{\mathit{long,min}}\)

Output scale#

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

Reliability#

Comparable to \(a_{\mathit{long,req}}\)

Validity#

Better than \(a_{\mathit{long,req}}\) [Zheng2019], depends on \(a_{\mathit{long,req}}\) and \(a_{\mathit{long,min}}\) estimate; suited for inter-vehicle comparisons; no empirical analysis available

Sensitivity#

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

Specificity#

High for humans, as braking is often preferred by human drivers [Adams1994]; strongly depends on \(a_{\mathit{long,req}}\) and direction of \(a_{\mathit{long,min}}\) estimation