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10th May 1974, Page 50
10th May 1974
Page 50
Page 50, 10th May 1974 — God bless you AASHO
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Which of the following most accurately describes the problem?

NUMBERS may or may not have the occult properties sometimes ascribed to them, but they can exercise a powerful influence on those of us who are susceptible. After we have read, in the recent paper by two research engineers from the Transport and Road Research Laboratory, that a 40,000 lb axle can do 115,000 times as much damage to the road as a 2,0001b axle, we expect every heavy lorry that passes to make a hole in the ground and disappear into it. When this does not happen, the conclusion is either that the 2,000 lb-axled vehicle is very light on its wheels or that somebody has not got his sums right.

The calculation is in line with the conclusion in the American AASHO test in 1962, that the damaging effect of axles is roughly proportional to the fourth power of the axle load. It is a pity that the word "roughly" has had to be introduced. Without it, the formula has the persuasive simplicity of Einstein's celebrated equation, and in its more restricted context it has had a similar impact.

Previously there was general recognition that the heavier a vehicle the more stress it caused to the road surface. Nobody suggested that the degree of stress increased at so catastrophic a rate.

Other people seem less eager than the British to accept the conclusion. Within the EEC there is still a hope, although a waning hope, of agreement on a maximum axle weight of 1 I tonnes. Some of the countries have higher limits, and clearly regard their willingness to reduce their demand as a sacrifice, and not as a welcome step to prevent their roads from crumbling away. Few of them, however, have wanted a maximum as high as 40,000 lb.

According to the paper, an increase from the present British maximum of 10 tons to 11 tonnes (10.83 tons) would double the amount of damage to the roads. An increase to 13 tons would cut the life of roads by 70 per cent, unless at least £400m was spent on strengthening them. The cost would be between E100 m and £150 m over 10 years if the increase applied only to existing 10ton axles.

Government policy appears to be that the extra cost puts any idea of a weight increase out of court. The argument is not convincing if evidence is that the increase will bring economies and improve efficiency. Many subsidies cost a good ,deal more, and Mr Healey's use of VAT as a device for putting up the fuel duty has even breached the long-standing convention that commercial vehicle users and motorists must share their tax burden on equal terms.

Pay more

Allocation to users of the cost of road repair and maintenance, if it had to be done, would be through vehicle licence duties rather than the fuel tax. The argument from the AASHO test conclusions is that the users of heavy goods vehicle should pay more than at present even without an increase in the permitted maximum weights.

But perfect equality in fiscal matters, as in anything else, is impossible. So much depends on the extent, not merely of the use of a vehicle, but of its use when fully laden, or laden up to the axle weight limit. Other factors affecting damage to the road must include the speed, and the type of road surface, whether smooth or rough or pitted.

Statistics in the paper are based on the estimated damage caused by a "standard" axle of 18,0001b. A fiveaxle articulated vehicle is devised with 4 tons on the front axle and 7 tons on each of the others (a total of 32 tons). The damage would be the equivalent of 2.4 "standard" axles, less than the ratio of 2.7 for a rigid 16tonner with 6 tons on the front and 10 tons on the rear axles.

It all seems a far cry from the situation only a few years ago when the vehicle manufacturers, under the impression that they would be pleasing an expectant Ministry of Transport, showed themselves able and willing to go into production on the basis of 44 tons gross and 13 tons axle weight.

They would cope equally well with lower axle limits. Operators might find more difficulty, especially with certain loads. A rich field would be opened up for the enforcement officers and the weights and measures inspectors.

by Janus

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Organisations: Ministry of Transport
People: Healey

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