Mathematics of Road Safety
By D. A. WILSON
WRILE the " Q cars " in the county of Oxfordshire feature in the headlines, the most hopeful discoveries concerning road safety remain largely unknown to the general public. This is a pity, because road safety is bedevilled by opinion, unsub- stantiated by fact, and an emotional House, mindful of the road casualty rate. might at some time succumb to the pressure of this opinion. As a result road-users would find themselves burdened by further ineffective legislation.
Yet the facts about road safety are slowly being 1 rought to light, and the agency responsible is the road-safety section of the Road Research Laboratory. It works in collaboration with various authorities, and is itself part of the wider Department of Scientific and Industrial Research. Periodically its work is described in learned papers read before equally learned societies, but the public has little inkling of its existence, although some of its manifestations attract attention. Chief amongst these is the " zebra " crossing ; least conspicuous the rubber tube that one's car occasionally crosses on a main road (thus counting itself in the automatic traffic-counter to which the tube is attached). A third manifestation was in the form of recent legislation insisting that auxiliary driving-lamps on vehicles should be mounted at least two feet from the road surface unless they were used solely in fog or snow. This was due directly to the statistics gathered by the organisation to show that twice as many of these " pass " lamps caused dazzle as did normally dipped head-lamps.
But the aim of the Road Research Laboratory in road safety is loftier than these incidentals would suggest. It is, in the words of the Director of Road Research, " to attempt to gather evidence which will reveal the laws of nature goVerning collisions between moving and stationary bodies, human and vehicular." The work of the Department goes slowly on — slowly because Traffic data take time to collect and collate, and because money and man-power for such work are not unlimited. None the less, the Jesuits so far achieved are creditable, for the road-safety section k a post-war foundation, set up as a result of a recommendation of the Alness Committee on Road Safety in 1939.
The fact that it is situated at Langley Hall, near Slough, means that Slough features largely in many of the statistics. But Slough is typical of very many towns, and within wide limits what goes for Slough will go for most of the country. Thus it is a shock to find that while the number of cars involved in personal-injury accidents per million vehicle-miles was 7.5, commercial vehicles 7.0, buses 6.5, and bicycles 8.6, the number of motor-cycles involved was 23.4. Here is the reason for the Government's new concern over the motor-cyclist.
And here is the reason why the Highway Code advises the pedestrian to walk facing the oncoming traffic. Only 34 per cent. of pedestrians walking on roads without footpaths, in daylight, face the traffic, leaving 66 per cent. with their backs to it. But only .27 per cent. of those injured were facing the traffic, while 73 per cent. had their backs to it.
Some of the findings confirm common-sense supposition, but the confirmation is necessary Far-reaching measures should not be undertaken unless they are supported by something stronger than the argument " It stands to reason," and the ultimate results achieved by the Laboratory are likely to lead to far-reaching legislation. Thus, although most of us are conscious of the dangers of wet weather, it is better to know that the wet-road accident-rate is 20 per cent. higher than that of the dry road. It would also be better if more local authorities were aware that the resurfacing , with non-skid material of part of the Finchley Road, in London, .reduced skidding accidents from 30 to two over a year, and wet- Weather accidents from 31 to five. This is a typical " before and after" result obtained by the Laboratory.' It is safe to say that if the road-safety section of the Laboratory had been in existence before the war the " Belisha crossing " would never have come into existence, but the scientists have done their best to reduce its inefficiency. The result, visually, is the zebra marking, which has had a quite encouraging result in compelling observance of crossing rules. What the public needs to be told, if possible in some easily absorbed manner, is that it is only one-third as dangerous to cross the road at a light-controlled cross- ing with a central refuge as it is to cross at random. The risk at such a crossing is only 1: 3,000,000. Risk rates have been calculated for other forms of crossing as well, although the uncontrolled pedestrian-crossing figure is disappointingly near that of the random point (0.89:1.0). The lack of value of opinions on road safety is well illustrated by the pedestrian-crossing work. It is frequently said that crossings need an advance warning strip, in order that the pedestrian shall not begin to cross if he sees that a vehicle has passed the warning strip. This is proved unnecessary by the con- siderable intelligence shown by the pedestrian, who has been proved capable of good judgement of speed and distance. Thus 72 per cent. of pedestrians will cross in front of a vehicle travelling at 5 to 10 m.p.h. and 60 feet away, but only 24 per cent. will risk crossing if the vehicle is travelling between 20 and 25 m.p.h.
The more intractable sources of road danger concern vehicles. So long as car-owners are at liberty to adjust their own lamps dazzle will persist, and the Laboratory sees polarised light as the eventual answer to the dazzle problem. To braking there is a limit ; mathematically it is g, a deceleration of 32 feet per second, and modern brakes, plus a good road surface, already approach it as nearly as is safe for the occupants of a vehicle. Tyres have not, however, reached perfection in the assistance that they give to braking, for quite surprising variation has been found between one tread-pattern and another, although in theory there must be an optimum pattern for quick retardation and resistance to skidding.
The road itself, and the traffic-flow upon it, have been the objects of much study. Rural junctions, a source of many accidents, have been developed from earlier work by the then county surveyor of Oxfordshire into a design that is considered to be the safest at present obtainable, and the Ministry of Transport is co-operating in the conversion of an actual junction to the suggested pattern. Meantime investigation goes on at typical. junctions (in one case by barrage balloon) and in controlled experiments at a disused airfield in Berkshire. Various road-suppositions have been. con- firmed. Twin tracks are safer than roads not centrally divided ; and accidents per million vehicle-miles increase with degrees per hundred feet of road curvature until, at a figure of 15 degrees and over, they reach 14.9. The comparative figure (0 to 1.9 degrees of curvature) is 2.6.
But traffic congestion presents the most gloomy findings. Over a wide network of London streets surveys have been taken at regular intervals since September, 1947, during which period the mean total flow in vehicles per hour has increased from 14315 to 1,345. This has resulted in the journey-speed decreasing from 11.3 m.p.h. to 10.9 m.p.h.—a cynical comment on cars that are capable nowadays of 80 m.p.h.—and the Director of Road Research has stated that an increase of traffic over these streets of 50 per cent. would reduce average speeds to the prohibitively low figure of 5 m.p.h. In other words London would choke- to death, and what is true of London is true of other cities that are equally congested.
To see where all this work is leading it is necessary to go back to the aim already expressed—the determining of- natural laws governing collisions. If the work goes on with equal success in the future it is possible to visualise an equation in which the number of accidents on a given stretch of road will equal a number of other factors, mathematically related to each other, that, are to be found on that stretch of road. It will then be possible to deter- mine the influence of each of these factors, and then, when money is once more available for road measures, a start can be made with the elimination of the most decisive. Moreover, that beginning can be made with the cirtain knowledge that, this time, the money is being spent in the best possible way.
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