Ac~'id. Anal & Pre~., Vo[ 10, pp. 61-67 Pergamon Press 1978 Printed in Great Britain

T H E D R I V E R A S C A U S E O R V I C T I M I N

V E H I C L E S K I D D I N G A C C I D E N T S

RICHARD A. OLSEN

The Pennsylvania State University, University Park, PA 16802, U.S.A.

(Received 1 June 1977, received for publication 3 November 1977)

Abstract--The implication, if not the evidence, that drivers are the principal causative factor in vehicle skidding accidents is widespread. This conclusion is discussed and refuted. There are several potential sources of information or knowledge which might allow the driver to prevent skidding, to avoid initiating a serious skid, to detect an incipient skid, and to control a skidding vehicle that have not been explored or developed. With the current lack of information supplied them, drivers are more often the victims of chance conditions rather than the causes of preventable losses of control. More comprehensive models of driver potential as a controller are called for, and a research program is outlined for developing the types and quality of information which potentially are available for better system performance.

A report on the study of accident causation over a five-year period on the Pennsylvania Turnpike [1954] was cited by Forbes [1959] in a paper given at the First International Skid Prevention Conference. It indicated that skidding or "inadequate coping with roadway condi- tions" was involved in from 25% to almost 50% of the accidents. Similarly, Giles and Sabey[1959] concluded that 27% of injury accidents on wet surfaces and 82% on icy surfaces involved skidding. Mills and Shelton [1959] found that 41% of rural accidents in Virginia in 1957 were reported to have involved skidding. Although most of these researchers made it clear that they perceived most such skidding as a breakdown in the overall system composed of the driver, vehicle and environment, it is tempting to blame the driver for the failure to "control the vehicle". Indeed such an assumption is implicit in the statements and official actions of legislators and traffic enforcement agents. There is little to indicate that the perception of this problem among those influential in fund allocation has changed significantly in the two decades since the first skid conference. No more than five years ago, a state justified the deletion of a driver research facility from its capital budget after a committee had concluded that there was no evidence that lack of driver skill was a significant factor in the causation of accidents. Presumably it was disregard for the laws or gross ignorance that caused that state's quarter million reported accidents that year.

From the point of view of the human factors professional whether or not the driver could have done something which would have avoided a skidding accident is irrelevant; the question is whether the driver had the kind of information which could be expected to lead him to conclude that there was a skid hazard. This question, if answered in the negative, virtually eliminates the driver as the "cause" of skidding accidents. At best we must plead ignorance and attribute most accidents to "chance" which is a more defensible cause than "driver error" in our present state of knowledge. As long as locations which are recognized as having "high" frequencies of accidents involve only an extremely small portion of the vehicles passing that site, the few that are involved in accidents hold the key: How are they or their drivers different from the vast majority of the traffic which is not involved?

A major deficiency still remains in that there has been no definitive study of what drivers know about skidding and slipperiness, or about their behavior with regard to wet roads and skidding. Although a primary factor affecting a driver is experience, it is only one element to be considered. In theory, driver skill and experience are maintained above some minimum level by official testing or training requirements for licensing. In practice this is not being achieved, in my opinion, or if it is, the minimum is too low. While we cannot change the driver's psychological or physiological conditions at any given moment, we can make greater efforts to show the direct effects of speed in wet road driving, and we can change the environment and the vehicle to provide stronger cues to proper behavior.

There also seem to be some learning effects from interaction with other drivers, since it has been observed [Hanscom, 1976b] that drivers tend to react to a situation differently depending

62 R.A. OLSEN

on what they observe other drivers doing. Continuing on-road education is a concept that has not been considered carefully. For example, while police activities are a potential driver education medium since police cars are present regularly in the traffic stream, they are often observed making the same suboptimal maneuvers that many "average" drivers make. The police generally have not developed a broad enough view of driver interaction and are not trained to the extent necessary for providing a good example to other drivers. Experimental active signs which inform drivers while the condition exists that they are following too closely or exceeding the speed limit [Hunter, Bundy and Daniel, 1976] have been shown to be practical and effective. Traffic error feedback using actual observed behavior has been effective in reducing driver errors in urban settings [Hutchinson, Cox and Maffet, 1969; Ben-David, Lewin, Haliva and Tel-Nit, 1970], but no practical programs have been implemented using these concepts.

The point has not been driven home through public education that at higher speeds some wet road surfaces can have the same coefficient of friction as ice, or that even the slightest swerve, brake application, or acceleration can trigger a skid on a wet surface at high speed. Some kind of on-road driver education should be devised for the skidding problem as well, beyond the useful but limited "slippery when wet" signs [Hanscom, 1976b].

Worn or underinflated tires are a common contributor to loss of control and skidding. Over 55% of the passenger cars checked had one or more "seriously underinflated" tires according to one report lB. F. Goodrich, 1977]. With the advent of radial tires which seldom blow out, and self-service gasoline stations where tires are not checked and an air hose and gauge may no longer be provided, there is little likelihood of this problem correcting itself.

Drivers routinely perform maneuvers which increase the requirement for braking and side forces considerably beyond that which would be necessary with alternative behaviors. This has been demonstrated by Olsen and Hostetter [1976], but there have been few, if any, serious attempts at shaping driver behaviors toward these more desirable types which have inherently lower friction demands.

Since much of driver behavior takes place in patterns carried out by well formed habits, conservative driver behavior probably must be shaped for all driving. If consistency were a conscious goal in roadway design, so that drivers could respond only to cues provided by the circumstances, there would be less need for developing high skill among drivers and less dependence on the driver's judgment as to when a slower speed is necessary [see, for example, Olsen and Meyer, 1969], The slowing should be in response to something perceived rather than to some rule about "wetness". Not only do drivers drive mostly without conscious attention to details of their behavior, but cautious behavior can be invoked only after a hazard is perceived. The perception of hazardous conditions is one of the main difficulties, and it is not logical to train operators specifically for skid control if the system does not inform them when there is an impending skid. Specifically, the vehicle and the environment are not conservatively designed to provide knowledge or feel of skid situations through visual inputs from the road surface and edges, through the steering torque feedback, through the engine and tire sounds, and through road noises and vibrations.

Such inputs are not always considered in analysis of the control functions. For example, a paper given at the Second International Skid Prevention Conference in Columbus [Hanscom, 1977] discussed some of the previous human factors work on skidding. One list of physical parameters as sensed by the controller (driver) contained five items: velocity, velocity change, acceleration,t headway and headway change [Rockwell and Snider, 1968]. This model did not include control feel and auditory cues. While there is no definitive evidence that these cues are useful or not useful, they have potential usefulness and are felt by some to be used now in subtle but influential ways. These cues may be used only by the most skillful drivers or only in specific situations, or they may be available only in certain types of vehicles. If so, the tasks of the driver educator and vehicle designer are clear.

There are other attempts at modelling driver-vehicle-roadway interactions which have similar gaps. For example, Hankins et al. [1971] described communications system to the driver as divided into "formal" and "informal". The formal category included signs (informative, directions, warnings), signals, and markings (edge lines, arrows, striping). The informal listed

tVetocity change refers here to change in forward velocity, while acceleration refers to lateral motions.

The driver as cause or victim in vehicle skidding accidents 63

geometrics, guardrails, delineators and alignments (horizontal, vertical). A more comprehensive listing, in my opinion, would relabel these two main categories to make them "formal-visual" and "informal-visual". Under the second heading I would add the behavior of other drivers as an information source, and the appearance of the road surface (sheen, previous tire tracks, color, puddles). I would also add a third main heading, "Other", to include tactual (steering feel, vibration of the controls), auditory, and acceleration. Missing in most models are considerations of the potential of vehicle design and driver training for influencing those driver habits and actions which might increase the number of swerves, jerks, or breakway maneuvers which, in turn, increase the probability of a skid. The sensitivity of brake controls has not been set through an optimizing process, nor is it variable so that it can be selected (except through choice of model) to meet the capabilities of individual drivers. A complete analysis of the driving task in a potential skidding situation remains the missing element for a skid reduction program.

When the analysis of information sources is complete, there will undoubtly still be a need for informational signing. A sign was once proposed with the legend "Road Ahead Deceptively Slippery". Since drivers work on perceptions of hazards rather than perceptions of signs, this kind of sign actually might be quite effective because it is telling the driver that the cues present are not sufficient by themselves to indicate the extent of the hazard. It would be better, of course, to increase the cues in some natural or artificial way so that the direct perception of slipperiness is accurate. This could be aided through signing, but the literature reviewed by Hanscom [1977] reminds us that there is still a lack of knowledge in the area of the design of warning signs for effectiveness: an area for further research.

There also seem to be accumulating indicators that some drivers who violate the instructions of signs do so because they do not see the signs [e.g. Johansson and Backland, 1970; Hakkinen, 1965: Hanscom, 1976b; Reiss and Robertson, 1976; Watts, 1977]; i.e. they really did not perceive the signs (or they did not make any attempt to look for them). Also, since the more obligatory a sign is, the better it is perceived [according to Johansson and Backland, 1970], there seems to be some filtering mechanism going on in perception which argues that certain types of signing should be much more carefully used. Although considerable effort has gone into designing signs for effective transmittal of information [e.g. The Manual on Uniform Traffic Control Devices, 1971], perhaps signs which instruct the driver on what do so, avoid, or look out for should be still further differentiated from signs which provide general information so that the chances of a driver filtering out and ignoring signs which do require action or precautions are lessened.

There still has been no systematic human factors study of skid accident causation. Some data are accumulating with implications for this systemized analysis from sources such as improved accident investigation and reporting techniques, and studies of driver response to signs [Hanscom, 1976a], but a definitive study of the potential for skid avoidance and skid control in the general driving public presents many difficult conceptual problems and practical limitations in testing and relating all of the relevant variables.

An example of the difficulty is that of Richie [1972] who showed the driver's choice of speed approaching a curve is .related to the perceived lateral g forces. However, there is little known about how the driver makes these g-force estimates [Richie, McCoy and Welde, 1968], and how manipulable these perceptions might be with such things as seat design and vehicle suspension changes. Drivers also commonly brake while cornering, enhancing the probability of a skid. In most situations the driver has the option of braking while cornering or of braking to the desired speed first and then entering the curve. This provides more efficient and controlled braking and also reduces the frictional demand on the vehicle-roadway interface by allowing more balanced lateral forces among the wheels and suspension system as they react with the road surface. Drivers seem to perceive the two-step process as an unnecessary inconvenience and loss of time. It truly is unnecessary for most conditions, but enhancement of the feeling of loss of control, probably through suspension changes to increase roll and side forces, could be used to reduce the incidence of actual loss of control by encouraging greater general conservatism in driver behavior. Whether this would be perceived as an "uncomfortable" car, affecting its sales, is one of many related questions. It is doubtful that driver education in the traditional sense has any promise of influencing the cornering behavior of motorists without more tangible inputs from the vehicle or roadway.

If there is any doubt that different vehicles tend to react differently in actual skidding

64 R.A. OLSEN

situations, a brief viewing of some of the films taken on 1-270 near Washington for the Blatnik Committee investigations [see Committee on Public Works, 1973, page 32] should dispell it: most of the small rear-engine vehicles spun in one direction, while heavier front-engine vehicles spun the opposite way. No formal study of these characteristics and their effect on driver skid performance has been done, to my knowledge.

Another potentially useful skid control technique related to driver perception of curves is suggested by the work of Rockwell and others [e.g. Mourant, Rockwell and Rackoff, 1969; Rackoff and Rockwell, 1973 and 1975]. If it is true, as this work suggests, that driver eye movements tend to fixate the edge line of a curve before entering it, further attempts should be made to manipulate edge markings to change the perception of the curve [see Shinar, 1975]. Drivers, when asked, perceive many potential skid situations by the sharpness of the roadway curvature and by the driving behavior of other motorists. They also fail to see skid hazards in many situations. However, a sign telling of the presence of the skid hazard is very seldom made use of by drivers unless it is unusually conspicuous, with flashing lights, etc. [from Hanscom, 1976b].

Many studies of conformance to speed limits have demonstrated that vehicle speed regulation by static and even variable-messages signing, without obvious enforcement efforts, seldom is very effective. However, signing with tailored instructions telling the driver exactly what to do, such as the advisory speed limit signing in the California curve study [Hammer, 1968] does appear to be effective. Since additional size implies serious intent of the signing, oversized curve signs may be necessary for especially bad situations where the unfamiliar driver is most likely to need special alerting techniques. It is unfortunate, but true, that many signs are perceived as not having "serious intent". They are often the result of attempts to avoid liability suits where a road may be hazardous at times and no effective solution is readily available, or where previous accidents or complaints have suggested that "something" should be done.

Where signing is necessary to warn of skidding hazards which are present only under specific conditions, variable message signing seems a logical approach. However, regulatory variable message signing, for example to reduce speeds during wet weather, fog, or night conditions, poses many problems including reliability. Once a variable regulatory speed limit or hazard alerting system malfunctions or gives misleading information, the legal implications are serious. Since variable signs seldom look exactly like standard signs, the driver's expectations are not fully met, and misunderstanding, lack of confidence in the sign, and differences in conspicuity may pose further problems.

One thing about signing seems to be reasonably clear: drivers should be given clues as to what is ahead, telling them what to expect, not telling them to "be careful", and not giving overly conservative messages. Enhancement of perception or enhancement of specific en- vironmental cues is likely to be more effective than word signs. The positive guidance concept [Alexander and Lunenfeld, 1975] is developed from this approach: alternative actions must be clear; the apparent attractiveness of each alternative should be related to the probability of a successful outcome through the provision of adequate information; and cues which tend to provide false or misleading information are minimized or clearly countered by other strong, reliable cues.

This kind of approach can be extended into the field of skid prevention and control. While currently the driver cannot obtain any reliable assessment of slipperiness from the appearance of a road surface, according to Hanscom [1974], that remains a potential information source. Clearly the intent of "slippery when wet" signing is to tell the driver that he cannot trust the appearance of the roadway ahead under some conditions. Whether or not it is feasible to develop a roadway surface that reliably signals its wetness or slipperiness has not been carefully explored. Certainly there is little likelihood of providing higher friction surfaces uniformly on all roads. The signalling of relative hazardousness would still be desirable in specific instances.

The skid school approach, though intuitively attractive, still has not provided the kind of information necessary for devising a cost effective skid remedy. In the skidding situation, it seems clear that a large portion of the public now expects to react with panic and that this panic might be reduced or eliminated by some type of training, simulation, or, less likely, by

The dr iver as cause or vict im in vehicle skidding acc idents 65

information or demonstration alone. Widespread advanced driver education has been promoted but it has never proved that it would be effective. Of course, the same statement continues to be made regarding all attempts at educating drivers. Once again, it seems necessary to provide additional sensory cues to drivers on the roadway to assist them in predicting a potential skid. Little has been done in a definitive way to find out what cues motorists actually use in detecting or avoiding skidding.

If driver education is improved as Forbes [1959] suggested, it has to be done in a cost effective way and again, there is little information that driver education is effective in reducing accidents of any kind. Skid training in actual emergency situations is expensive, and some definite incentive would be required to induce drivers to take on this extra cost, once it has been established that there is a reasonable benefit in reducing losses and once facilities are available. The failure of drivers to make use of seat belts that are already available tends to make one think that such an approach is not very promising.

It seems that the greatest hope of reducing the incidence of uncontrolled, inadvertant skidding remains through human factors engineering, broadly defined, rather than through education or speed limit enforcement. While a four-locked-wheel stop may be the best tactic available in certain pre-crash moments, skidding is not intentional in the vast majority of actual occurrences. Many elements of a skid behavior model are already available, but the more subtle, integrating elements still nave not been defined. They are not going to be determined in casual observation but only after a series of basic studies. There are sensory inputs from auditory and tactual sources which have not been considered fully and have not been ruled out in definitive experiments. There are vehicle and personality differences which, without doubt, have considerable importance. The trend toward smaller cars may have a tendency to increase the feedback provided to drivers, while it also may have the disadvantage of increasing the likelihood of skids or other accidents and also the severity of the damage done in any accident [Dutt and Reinfurt, 1977]. Since we may soon have no choice but to go to smaller vehicles, the emphasis on crash-worthiness will be stepped up, but the driver's appreciation of the need to control speed and the enhancement of cues which signal an incipient skid seem to be the most fruitful preventive approaches. For this to be effective we need to know much more about what drivers can and do react to and how these cues can be controlled or enhanced to provide a lower rate of uncontrolled skidding incidents.

Clearly, this paper is a call for further research and subsequent implementation. Some of the elements of this program appear in outline form below.

I. Systematic human factors analysis of skid accident causation A. Roadway elements

1. Define skid-related cues available from the geometry, surface, edges and other physical features of the road environment.

2. Explore road design variables (e.g. superelevation encourages higher speeds). 3. Develop new sources of information (e.g. a surface that looks wet, reliably, when it is

wet). 4. Enhance cues that are already useful (e.g. edge lines denoting curve sharpness or length:

open-graded surface tends to be less slippery). 5. Explore the timing or correlation between cues signalling skid potential and actual

conditions of high skid potential. B. Vehicle elements

1. Describe cues to incipient skidding already available in various vehicle types (e.g. vibration, side force transmitted to driver, sound, control feel and reaction).

2. Describe vehicle behavior in various skidding situations for various vehicle design types (e.g. oversteer; understeer; weight distribution; front- vs rear-wheel drive).

3. Explore vehicle design variables outside the range of common current practice for cue-enhancement potential.

4. Define the maintenance aspects, especially for tires, which are important in skidding. 5. Develop new sources of information or cues from the vehicle that are related to skidding.

C. Driver elements

1. Ascertain the incidence of skidding as a causal factor in crash loss vs an incidental occurrence (e.g. panic braking is not always skidding, although alternative evasive steering is

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often a better tactic); from accident analyses, near-accident studies, and studies of driver capabilities.

2. Define the range of knowledge among drivers regarding skid causes, prevent ive measures (e.g. tire pressure), avoidance (e.g. path choice or speed), skid control (accelerator and brake modulation), steering strategies, panic stops and other intentional skidding.

3. Explore driver preparation: formal driver education, public education, on-road educat ion; skid theory and practice, schedules; perception of cues from training and from experiences: habit formation and modification; reeducat ion and behavior shaping.

4. Describe driver control: classified and restricted licensing; main tenance of physical and visual skills; retesting requirements and schedules; diagnostic and remedial testing, training, and practice of skid skills. D. Artificial information sources

1. Study static signing: relevance, utility, conspicuity, interpretation, relative driver con- formance, driver response variation.

2. Compare variable signs (on-off and variable message) for need, utility, and cost. 3. Study sign "noise" (extraneous or ignored signs); t iming and control; sign types and

classification; risk level appropriate for sign types; legal implications. 4. Explore novel ways of artificially increasing driver awareness of high skid potential. II. Synthesis of a more effective skid signalling and control system integrating all current

and novel elements to assure driver skill and knowledge are sufficient to meet the skid threat in the natural or cue-enhanced envi ronment .

III. Implementa t ion of elements from the improved system: develop means for bringing about necessary legislative, administrat ive, and judicial changes to make the new approach possible and put it into use.

IV. Evaluat ion of changes for initial and eventual influence in reducing skid-accident losses; possible related effects; costs; possible further improvement and research topics.

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