excessive speed and the intersection accident

8/9/2019 Excessive Speed and the Intersection Accident

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Excessive Speed and theIntersection Accident

Presented by Johan Joubert: Traffic Consultant


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The speed limit on all freeways in South Africa is 120km/h.

This is the general speed limit on freeways as prescribed inthe Road Traffic Act. Law enforcement can only start when amotorist exceeds 132km/h (10km/h grace allowed). InKwazulu Natal this grace is 20km/h, so Law Enforcement canonly start at 141km/h.

During peak hours, the average speed is much lower due tovehicle volume and densities. The problem with speedviolations is particularly extreme during off peak hours andespecially after the afternoon peak.

At this time the variance in speed, i.e. the difference betweenfast moving and slow moving vehicles becomes a major roadsafety risk.


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Research has indicated that the variance in the speed of vehicles on a particular segment of roadway is a measure of the distribution of actual vehicle travel speeds above andbelow the average travel speed. The higher this figure, themore dispersion there is of speeds, which contributes to thepotential for more crashes.


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A phenomenon of speed variance is that as the speed of avehicle increases beyond 15km/h above the average speed ona highway, the risk of it being involved in a crash increasesexponentially.

At 32km/h above the average, the risk of a crash has beenestimated at 11 times greater than at the average travel speed.


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The laws of physics say that a vehicle s speed energyincreases with the square of its velocity increase. As anexample, suppose the prudent travel speed on a roadway was70km/h but that a through vehicle was traveling at 90km/h.

A simple computation shows that when a vehicle increases itsspeed from 70 to 90km/h, it has made a 77 % increase in itstotal speed energy.

Thus, an excessive speed of 90km/h on a 70km/h prudentspeed roadway results in much more speed energy availableto cause harm.


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Excessive speed reduces the ability to accommodate judgment errors of others.

As the vehicle speed increases, the ability to accommodateother motorists lessens correspondingly. There are severalsub factors that cause this and some of these are:

Each succeeding increment of increased speed requires a much larger increment in stopping distance.

F or example, on good road surfaces, an average car requires

only around 16m of heavy braking to reduce its speed from70km/h to 50km/h, but the same car must brake close to 23mto reduce its speed from 95km/h to 70km/h!


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The excessively speeding vehicle has less time to safely respond .

It takes about one second for an excessively speeding car toslow from 95km/h to 70km/h during emergency braking andanother second to go from 70 to 50km/h. Thus, the extrasecond taken to slow down from the excessive speed is oftenthe time that could have been used to accommodate an error of another motorist.


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The ability to visually monitor the road for collision

threat situations decreases with increased speeds.

Driving is largely an activity of tracking and obstacleavoidance. As our speed increases, this naturally reducesthe time available for both tasks during our approach to an

intersection.

Also, in some situations such as traveling through curves, wemay have to devote larger increments of time to tracking thanat slower speeds.


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The ability to safely steer away from the collision threat decreases with increases in speed.

At lower speeds, we can make strong steering changes withrelative safety. At much higher speeds, these steeringchanges typically have to be reduced to avoid total loss of

control.3 . Excessive speed can contribute to judgment errors byother motorists.

The roadway doesn t have to be totally devoid of traffic for usto feel relatively safe in crossing or turning. When we look for clearance to enter or cross, we subconsciously look for vehicles that are within our collision threat zone


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Each of us develops our own version of a collision threat zonewhich is nothing more than an envelope of clearancenecessary for us to safely make a turning or crossing

maneuver.

However, a vehicle traveling at a highly excessive speed maybe a strong collision threat but lie just outside this col lisionthreat zone envelope.


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Thus, a motorist may begin a crossing or turn and suddenlybe confronted with an impending collision. Sometimes thecrossing or turning motorist may have an opportunity to avoidan accident, yet for some reason fails to abort the maneuver.

This may occur because the motorist was confronted with a

strong violation of expectancy. Suppose, in the originalviewing for clearance, no vehicle was within the collisionthreat zone.


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Now, if a speeding vehicle suddenly appeared nearby duringan early phase of the turn, this vehicle may not immediatelyregister as a collision threat in the mind of the motorist.

When things are not supposed to be there, the mind tends notto immediately accept the reality of their appearance. Also,when something strongly violates our expectancy, it can takelonger for the mind to accept and devise anappropriate response.


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In summary

An excessive approach speed can have several adverseinfluences on the typical intersection accident.

It adds much more dangerous energy to the situation,reduces the ability of the speeding motorist to accommodateerrors, and can contribute to judgment errors of the crossingor turning motorist.


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B IBL IOGRAPHY

1. AA RSA Road Traffic Safety Foundation, Annual Traffic Safety Audit, 1998.2. E ffects of Raising and Lowering Speed Limits, US Department of

Transportation, Federal Highway Administration, October 1992.3. G uidelines for Setting Speed Limits, Technical Report RV/ 19 , National Institute

for Transport and Road Research, CSIR, February 1986.

4. M aximum Posted Speed Limits for Passenger Vehicles, Insurance Institute for5 . Highway Safety Highway Loss Data Institute (HIS-HLDI), October 2001.6. South African Road Safety M anual, Final Draft, M ay 1999.7. Report 55: A Decade of Experience . Transportation Research Board .8. The Effect of Speed, Flow and G eometric Characteristics on Crash Rates for9. Different Types of Virginia Highways, Nicholas J G arber, PhD . Virginia10. Transportation Research Council, University of Virginia, Charlottesville,

Virginia, January 2000.

excessive speed and the intersection accident

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