[SystemSafety] Small but useful Detail on Road Stopping Distances

Fri Jul 31 08:47:15 CEST 2015

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On 2015-07-30 08:18 , David Haworth wrote:
> On 2015-07-29 15:58:35 +0200, Peter Bernard Ladkin wrote:
>> The traffic law in Germany stipulates a reaction time of 1 second.
> 
> The "halber Tacho"* rule for driving on the Autobahn approximates to a 2 second reaction time
> (since your safe distance behind another car doing the same speed is just your reaction time, 
> all other things being equal). Or one second plus "engineering tolerance" ;-)

2 seconds is a decent enough estimate, but it's not what the law generally uses.

There is a ruling from the Supreme Court (Bundesgerichtshof) that the reaction time, including
brake activation time, is about one second for unexpected events ("Bei einem unvermuteten Vorgang
beträgt die Reaktions- und Bremsanprechzeit eine "knappe" Sekunde, s BGH NJW 00 3039, Ha VR 80
685" Hentschel, Strassenverkehrsrecht (Road traffic law), 37. Auflage (edition), note 30 to StVO
§1). The law does recognise, though, that reaction times are variable (op. cit.)

Jagow Burmann Hess, Strassenverkersrecht (Road traffic law) has a diagram in the commentary to
StVO §3, note 13, which gives a diagram for stopping distances at various speeds, in which the
reaction distance is based upon a reaction time of 1 second. Source is given as the Verkehrswacht
Dortmund.

Here is a non-authoritative Internet source which also uses 1 sec:
http://www.verkehrsportal.de/board/index.php?showtopic=55805 and says "Bei diesen Faustformeln
wird eine Reaktionszeit von 1 Sekunde angenommen" ("In this rule of thumb, a reaction time of 1
second is used")

And here is that of course supreme authority Wikipedia
https://de.wikipedia.org/wiki/Sicherheitsabstand  "Die Reaktionszeit beträgt typischerweise eine
Sekunde" ("Reaction time is typically 1 second")

And here is the Polizeipräsidium in Frankfurt am Main also saying "on average 1 second" (second
page bottom)
https://www.polizei.hessen.de/icc/internetzentral/nav/9be/binarywriterservlet?imgUid=d7e717da-cb49-ff71-29c3-611142c388eb&uBasVariant=11111111-1111-1111-1111-111111111111

Here is the calculation taught at the Police Academy in Brandenburg by Kammann and Lederer
http://www.kay-kammann.de/mediapool/142/1422619/data/_3_StVO_-_Arbeitsblatt.pdf

The roadway engineering texts are a little less rigid.

The roadway engineering textbook Mensebach, Strassenverkehrsplanung /Strassenverkehrstechnik,
(Road traffic planning and technology) 4. Auflage, Werner Verlag 2004, Section 2.1.2 Bremsvorgang
(the braking process) says the reaction time  is 0,6 to 1,8 seconds, average 1 second.

The roadway engineering textbook Schnabel/Lohse, Grundlagen der Strassenverkehrstechnik und der
Verkehrplanung Band 1, 2. Auflage (Fundamentals of road traffic technology and traffic planning,
Volume 1, 2nd edition) says (p51) that the reaction time lies between 0.5 and 1.5 seconds, and
that the activation time for the brakes is a further 0.2 to 0.3 seconds and is often counted in as
"reaction time", so that gives 0.7-1.7 seconds or 0.8-1.8 seconds depending on which you take.

PBL
Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de