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Influences that Make for Accidents

15th December 1950
Page 58
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Page 58, 15th December 1950 — Influences that Make for Accidents
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Which of the following most accurately describes the problem?

"'IF the results of tests carried out are representative, there is room I

for much improvement in the design of the brakes of heavy commercial vehicles. Experiments with commercial vehicles having good brakes show that the small and medium lorry can stop almost as quickly as the best cars, although, in some cases, the braking performance is 'rather lower when the vehicle is fully laden.

"In the case of a few heavy commercial vehicles having power-operated brakes, it has been found that although they arc capable of achieving a relatively high maximum deceleration, they have a low average deceleration, especially when fully laden. This has been traced to the comparatively slow build-up in the power-operated system."

This criticism of commercial-vehicle braking was made by Dr W. H. Glanville, director of the Road Research Laboratory in the Department of Scientific aed Industrial Research, in the second of three Cantor lectures delivered to the Royal Society of Arts on each of the past three Mondays.

In his first lecture, Dr. Glanville described the historical background of the Road Research Laboratory and explained how its scope had been amplified to include research into traffic, road layoutand. vehicledesign, as well as into road-surface qualities.

Accidents Cost £,100 m.

At the 1946 value of the £,?oad accidents were costing the country £100 m. a year, and the work of the Road Research. Laboratory was described as being to reduce accidents by any means consistent with national economic wellbeing and the democratic way of life.

Dr. Glanville dealt with accident rates for different types of road vehicle and weather, from which it could be seen that they rose until noon, were steady until darkness fell and then rose again. Throughout wet days, the general accident rate was shown to be 20 per cent. higher than on dry days.

"In principle," said Dr. Glanville, "we know how to make road surfaces which are not slippery." He quoted an example where a test strip of woodblock road was resurfaced with a nonskid material, and in the following year the total number of accidents occurring on the new strip of road was seven,

B24 compared with 61 in the previous year.

Frequency of accidents, he continued, could be greatly reduced by physical improvement in roads. Improvement in vehicles would also help, but a great number of accidents could be avoided by greater care or skill.

Turning in his second lecture to the vehicle .and the road surface as important factors in the study of road accidents, Dr. Glanville said that vehicles, like drivers, had personal qualities and idiosyncrasies—one would have excellent manners and another would be awkward and difficult. Ease bf manceuvring, stopping and accelerating did not depend only on the vehicle, but also on the characteristics of the road, and particularly on the properties and condition of the road surface. At night, efficient, properly adjusted head lamps were important, and these were helped by adequate street lighting and by a road surface chosen to make objects more conspicuous.

The Dazzle Problems

Post-war reports of Army accidents indicated that vehicle defects played a part in 51 pet cent, of them, and dazzle from head lamps was said to have occurred in 13 per cent. of night-time accidents. The dazzle problem could he much reduced if drivers would see, individually, that their tights were properly adjusted.

The seeing distance with dipped lamps of the kinds in common use, even when carefully aligned, could be very small, particularly when the object to be observed was dark. The distance at which the darkest clothing could be seen with ordinary head lamps might be only about 50 ft. to 60 ft. This was less than the etopping distance from 30 m.p.h. for a vehicle with first-class brakes driven by a person with quick reactions. Seeing distance decreased with speed, whilst the stopping distance increased rapidly.

Turning to the investigations made by the Road Research Laboratory into vehicle braking, Dr. Glanville said that drivers appeared to be disinclined to brake as rapidly when stopping from high speeds as when drawing up from 30 m.p.h. Stopping distance was the sum of the braking distance and the distance related to the reaction time of the driver.

In a moving vehicle, the shortest reaction time was about half a second, and about half this time was occupied in transferring the foot from the accelerator to the brake pedal. At 30 m.p.h., a distance of 22 ft. was travelled in half a second.

Ability to resist skidding was an important aspect of vehicle and road performance, and at the present time .about 3 per cent, of all reported accidents were attributable to skidding on wet and dry roads.

On wet surfaces the difficulty was that a liquid film covered the road, acting as a lubricant and hindering direct contact between the tyre and the road surface. To obtain good performance on a wet surface, the contact area must have a number of smaller areas with spaces between them into which the liquid could escape. This could be achieved by constructing a road with an open-textured surface or by the use of tyres with a patterned tread.

The hardness of the rubber in the tread of the tyre also had an important influence on skidding resistance, which rose in relation to the hardness of the tread rubber. A recent survey showed that about 30 per cent, of the vehicles examined had smooth treads on two or more of the tyres. Good treads were essential for safety.

Better Road Lighting Needed A study of the effect of street lighting on accidents had revealed that the fatalaccident rate in the dark was more than twice that in the light, even in built-up areas. It appeared that road lighting was not yet good enough.

In lighting a street, the engineer aimed to make the road surface as bright as possible, so that pedestrians or other objects should stand out as dark silhouettes. Vehicle lighting worked on the opposite principle and made the object stew' out light against the darker road surface.

Some departures from the common types of kerb might be advantageous. Kerbs of special form have been designed to achieve greater contrast. By cutting away the upper surface of the kerb at regular intervals, a number of faces is presented more or less at right-angles to the beam of the head lamps and the kerb is therefore more .easily picked out.

Some of the problems of traffic in the mass, and particularly that of keeping vehicular traffic on the move, were discussed in the third lecture. .

Some of the apparatus and techniques developed for measuring the various properties of traffic were described. A number of traffic-counting devices was developed, including portable ones. Speed recorders, more convenient than the old " traffic-cop " method, were also used, in particular an American one, the Speed-meter, which used a beam of very-high-frequency radio waves. Traffic distribution and behaviour at crossroads were determined by means of aerial photographs. Lately, some had been taken from a captive barrage balloon, flown over crossings under investigation.

Observation of Traffic

Observer teams in test cars had proved useful in studying traffic behaviour. They were filtered into traffic, one at each end of a selected circuit, and data had been collected on the traffic using a particular road, of standing vehicles and of the number of vehicles which turned off the roads.

Surveys taken over a period of three years had shown that private cars had increased as petrol had become more plentiful, that commercial-vehicle traffic had maintained a similar level over the three years, although increasing slightly in 1949, whilst bus and taxi traffic had declined. This was probably because

journey speeds. were highest when traffic now was least, and the increased flow of traffic reduced the number of times the buses and taxis were counted.

Records taken of time lost at road junctions revealed that, on an average. about a quarter of the journey time in Central London was wasted on delays at intersections. Taking into account time spent in slowing down and accelerating, it was estimated that such delays accounted for a third of the period of a journey.

The importance of maintaining traffic lights at their highest efficiency was shown by experiments. Proper linking of the automatic signals on an important West End thoroughfare reduced delays by 40 per cent. and journey time by 8 per cent.

Discussing the effect of one-way streets and no-waiting regulations, Dr. Glanville said that observations taken before and after the introduction of one-way working showed an increase in traffic speed, although in some cases journey time was lengthened. NO-waiting regulations were more effective in Central London than in the suburbs.

In the year following their introduction, the number of injuries sustained throughout the Metropolitan Police district had increased by, 8 per cent. In no-waiting areas, however, accidents were reduced by 31.5 per cent, during no-waiting hours and by 18.4 per cent. outside these periods.

It was clear that substantial improve

ment could be achieved by the radical reconstruction of existing highways, c r by a reduction in the number of vehicles. One-way streets and no-waiting regulations were alternative methods of improving the position, and it had been made clear by the investigations that traffic speeds could be further increased if alt waiting vehicles could be eliminated. The traffic capacity of roads had been estimated. A 50 per cent. traffic increase in the Strand would halt movement.

Speed Limits Ignored

Discussing average speeds of vehicln on different classes of road, he said tha. in Central London, private and commercial vehicles maintained a similar average speed. Outside built-up areas. private cars travelled 5-9 m.p.h. faster than commercial vehicles. Slippery surfaces in urban areas reduced traffic speeds and were thus unsatisfactory both as regards accidents and traffic operation.

On examining the effectiveness of commercial-vehicle speed limits outside built-up areas, it had been found that vehicles limited to 20 m.p.h. averaged 27 m.p.h. Well over 90 per cent. exceeded the 20 m.p.h. limit_ The average speed of 30 m.p.h. vehicles was 30 m.p.h., but 60 per cent. exceeded this speed.

[Dr. Glanville's lectures will be printed in full in the Journal of the Royal Society of Arts.]


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