Re: [SystemSafety] Small but useful Detail on Road Stopping Distances

From: Mike Ellims < >
Date: Wed, 29 Jul 2015 14:31:21 +0100

A 9m reaction distance at 30 kph is around a 1.1 second reaction time which looking at the literature is faster than "normal" reaction times to events such as brake lights.

Green (1) gives the following values in a meta study of brake reaction times:

        high expectancy, little uncertainty, 0.70 to .075 sec (0.2 sec is movement time)

        normal, but common, signals such as brake lights, 1.25 sec
        response time for surprise intrusions about 1.5 sec (0.3 sec
movement time)

1.5 seconds at 30 kph is about 12.5m, at 50 kph 20.8m.

Other studies have found much larger reaction times, up to 4 seconds and Greens work has attracted a fair amount of comment in that it oversimplifies the issue and doesn't take into account all the factors. In particular some studies suggest drivers appear to do better than 1.5 seconds in urgent situations where time to impact is four seconds or less (1.3 seconds). However 9m and 15m still comes up short...

The upshot is that headway should be at least 2 seconds which is not good when most drivers have a measured headway of one second or less... thus Peter is on the money in terms of drivers not taking reaction times into account.

(1) Marc Green, "How Long Does It Take to Stop?" Methodological Analysis of Driver Perception-Brake Times, TRANSPORTATION HUMAN FACTORS, 2(3), 195-216.

-----Original Message-----

From: systemsafety-bounces_at_xxxxxx [mailto:systemsafety-bounces_at_xxxxxx Peter Bernard Ladkin
Sent: 29 July 2015 12:25
To: The System Safety List
Subject: [SystemSafety] Small but useful Detail on Road Stopping Distances


Hash: SHA256

There was something in the local newspaper on Monday about the increasing accident rate amongst motorcyclists in the state (North Rhine Westfalia). In Germany, troops of motorcyclists go out on the weekend on fast machines to indulge in a spot of boy-racing, although they don't call it that. A fair number of them come off on a curve and hit things. Then there are those who collide with oncoming traffic during an overtaking manoeuvre, also often where sight is reduced, such as in curves. Then there are those who just can't stop in time when something unexpected happens.

I have always liked motorcycles, have a licence and have owned a couple, but was always aware of my vulnerability. I don't ride at the moment, because I prefer expending energy while travelling (cycle) and don't like to have to cover myself up all over, especially my head, when it's nice weather.

Accompanying the article was a neat statistic from the local police. They reckon stopping distance from 30 kph is 9m reaction distance and 4,5m braking distance. From 50 kph it's 15m reaction distance. So, they observe, someone travelling at 30 kph is already stopped before someone at 50 kph has reacted. Obviously there is variation involved which is omitted, but the principle in the last sentence likely holds true per person and vehicle anyhow. Many people omit reaction time when thinking about how they stop, or they have a completely unrealistic idea of the role it plays. The speeds are chosen because they are two of the three speed restrictions inside urban areas. (The third is "walking speed" on designated "play streets".)

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

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Received on Wed Jul 29 2015 - 15:31:48 CEST

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