stoppingdistances

It’s worth repeating - stopping is just as vital as moving forward. No owner of any truck would like to be in an aircraft that runs out of runway on take-off. Why then do they let their drivers operate trucks that run out of stopping distance? If you are in trucking and do not understand the importance of stopping distance – the runway distance required to come to a halt – then why are you in this business asks our technical correspondent Dave Scott?

The analogy of an aircraft that fails to take off and a truck that fails to stop are intertwined. An aircraft takeoff failure is a cascade of events – too few hours between bottle and throttle, load, poor judgment and over-confidence, mechanical defects, insufficient power, badly conducted flight pre-checks, unfavourable weather conditions, inadequate training, surface rolling resistance and gradients. In the case of an aircraft, each influencing factor lengthens the take-off distance but in the case of stopping a truck, the closing gap just gets longer when it should be shorter. 

Stopping distances are critical

Trucks need distance to stop in an emergency but unthinking motorists on freeways take the gap and fill it. Five to 10 metres between a car bumper and a truck at 80kph is just not enough. 

Table 1 (below) shows a Utopian scenario at 80kph with everything in ‘perfect working order’ on a good surface with excellent weather/ visibility and using wide-awake drivers: 

• Scenario 1 is a sports car fitted with an ABS braking system. 

• Scenario 2 is a medium truck with vacuum/hydraulic brakes and no ABS system but everything is mechanically correct. However, braking efficiency is not as good as the sports car. 

 Medium trucks are driven like passenger cars and at 8,5t GVM, they can wreak havoc in an emergency due to the kinetic energy generated. The medium truck takes 40% longer to stop than the sports car. Do drivers really think of this? 

Up the stakes

Now to reality in Table 2. Let’s up the stakes to 120kph. Remember that all trucks under 9t GVM are allowed to travel at 120kph under Road Traffic Act Regulation 293 (1). 

• At 120kph, a perfectly-driven ABS equipped sports car takes longer to stop than in the scenario sketched out in Table 1. 
• Scenario 2 here introduces badly maintained brakes at 30% efficiency and a tired driver with a 2-second reaction time. The brakes have heat-faded and loadsensing is not functioning. 

At 120kph, the 8,5t GVM truck takes 124% longer to stop than the car. And how many 56t GCM rigs do you notice that far exceed the 80kph limit where the same laws of gravity apply? 

Driver reaction at 120kph is an absolute factor where one second's travel is 33,33m. But because there is no traffic enforcement at night, everyone likes to up the speed level when even excellent headlights on a dipped beam reach a max of 60m. The point is that on a dipped beam at 120kph at night, if you see it you will hit it! 

Research conducted on SA roads in 1995 is more relevant than ever 14 years later. A comprehensive report, 93/336 dated July 1995 titled ‘The influence of night driving and wet weather conditions on driver and vehicle abilities’, prepared for the Department of Transport (DoT) by BKS Inc. of Pretoria, comments that: ‘During the night, the frequency of collisions is three to four times higher than during the day.....darkness has an adverse effect on road safety, as both the severity and the frequency of collisions increase during this period.’ 

This is verified in a histogram published in the report covering a five year period titled ‘Association between frequency of collisions and light’. A fair estimate would increase the odds today – the chance of an accident at night is at least now four to five times greater than in daylight. 

Bring in the OHS Act 

If reaction times are so critical, why are so many driver controls in the cab - which is an OHS Act declared workstation - in such a sloppy mess? Poor reaction times resulting from driver fatigue are only compounded in loose linkages starting at the brake pedal and then all over a vehicle combination right down to slack adjusters that are sometimes fastened with wire. 

Is it not time to call in factory inspectors to enforce the conditions of the OHS Act in a truck cab? Road traffic enforcement agencies cannot seem to get this one right and perhaps there’s another approach to vehicle safety that we haven’t tried. 

It’s all about how much runway there is to play with. On the road, it’s a constantly changing scenario but for an aircraft, it’s pre-determined. You don’t take off when someone else occupies the airstrip and runway length is a set figure. Truck driving does not allow those parameters, so reaction times and stopping distances are therefore vital. 

We’ve seen Government ministers attend a mining accident funeral that is statistically small by comparison to the daily slaughter on our roads. Where are these same Ministers when there’s a multiple death road accident? 

I repeat: If you are in trucking and do not understand the importance of stopping distance – the runway distance required to come to a halt – then why are you in this business?