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Ski ding Accidents and Their Avoidance
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Lennart Strandberg
on Experimental Safety Vehicles, Gothenburg, Sweden
May 29 - June 1, 1989, pp 825-828
Reprint from
Skidding Accidents and Their Avoidance with Different Cars
Lennart Strandberg,
Swedish Road and Traffic Research Institute,
VTI
Abstract
In Swedish statistics from 1985 and 1986, the percentage of oncoming vehicle accidents is four times greater on
snowy/icy roads (16%) than on dry clear roads (4%). The
percentage rear end accidents is the same (6%) for both road conditions. Also the fatality numbers (364 people in oncoming vehicle versus 26 in rear end accidents) indicate that more emphasis should be put on instability and skidding problems (typical for oncoming vehicle accidents) than on braking distances and steering-while-braking
problems (rear end accidents).
Vehicle dynamics show that the cornering performance of the rear wheels must be superior to that of the front
wheels, if a car is to be stable at all speeds. This is illustrated
by some hydroplaning fatality cases, where the tyre pattern was much deeper at the front than at the rear. Since traction or braking forces reduce the side force, skid recovery will be
more difficult in a front wheel driven car, if the driver
depresses the clutch pedal before countersteering. Experiments with experienced drivers offered an explanation to why such a behaviour still is recommended in many driver training courses.
In uence on stability from vehicle design and equipment is being even more pronounced with antilock (ABS-)
brakes, 4 wheel steering, cruise control, etc. This must be
considered in research on vehicle black spots in accidents,
in vehicle inspection and testing, and in driver training programs.
Focus on Safety Relevant Properties in
Vehicle Technology Development
An increasing number of investigators claim that improvements of crash avoidance properties in cars do not increase the net driver-vehicle safety. For instance, the distinct improvements with antilock (ABS-) brakes in experimental driving tests found by Rompe et al (1987) were contradicted by accident records from real traffic according to Aschenbrenner et al (1988).
It has also been concluded that training of drivers on skid pads and in advanced evasive manoeuvers may increase the accident risk in real traffic. Glad (1988) found that drivers with skidpad training were involved in more accidents per mileage unit than comparable drivers without such training. However, controllability (steering and braking) performance had been emphasized with a disregard for the directional stability properties both in the tests by Rompe et al (1987) and in the skidpad training evaluated by Glad (1988). This may explain the paradoxical findings by Glad
(1988) and by Aschenbrenner et al (1988), since there are
evidence from Swedish accidents and driving experiments that poor stability may contribute to a much greater number of serious accidents than what poor controllability does. Therefore, the question on stability versus controllability may be decisive both of safety and of public demands on vehicle technology developments in the future.
In uence on stability from vehicle design and equipment
is being even more pronounced with ABS brakes, with
front/rear or four wheel drive, with cruise control, with four
wheel steering, with (prohibitions on) studded tyres, with increased differences in tyre adhesion between wet and dry
conditions, etc. This must be considered in research on
vehicle black spots in accidents, in vehicle inspection and testing, and in driver training programs.
Directional Stability Versus
Controllability in Accident Statistics
In official statistics of Sweden from 1985 and 1986, the percentage of oncoming vehicle accidents with personal injury is four times greater on snowy/icy roads (16%) than on dry clear roads (4%), SCB (1986) and SCB (1987). The percentage rear end accidents is the same (6%) for both road conditions. When considering the Swedish accident numbers from 1985 and 1986 to be representative samples of these accident types, Strandberg (1988) found that their difference in road slipperiness sensitivity was significant on the 0.001 level (chi square 173 and 253 respectively).
Perhaps it is contrary to common belief, but slippery road conditions are much more overrepresented in oncoming than in rear end accidents. See figure 1. This indicates that more emphasis should be put on instability and skidding problems (typical for oncoming vehicle accidents) than on braking distances and steering while-braking
problems (rear end accidents).
REAR END: 1 1 24 accidents ONCOMING: 1 044 accidents Ice Snow
Figure 1. Distribution of road surface conditions in two types of police reported accidents with personal injury, Sweden 1986. Data from SCB (1987).
Also the fatality numbers in multiple vehicle crashes are
dominated by oncoming accidents. According to
Swedish statistics from 1985 and 1986, oncoming
vehicle accidents killed 364 people, while 26 persons died upon rear end accidents. Figure 2 gives the division of fatalities during 1986 between different types of accidents between motor vehicles.
Superior Front Wheel Cornering
Performance Threatens Stability
A vehicle is stable if the resultant of external forces applies to the rear of the centre of gravity (figure 3). When aerodynamic forces are negligible, this condition is fulfilled in a car at any speed (and without demands on compensating 826
steering), if the cornering performance (or cornering
stiffness coefficient as defined by SAE, 1975) of the front
wheels is inferior to that of the rear wheels. See Strandberg (1983) and Strandberg et al (1983) for mathematical details.
331 killed people 1 986 in 256 accidents between motor vehicles
Others Crossing Exltlng Overtaklng Rear End Oncomlng 0 20 40 60 80 100 120 140 160 180
Nunber of killed people
Figure 2. Number of killed persons in different types of
accidents between motor vehicles, Sweden 1986. Data from SCB (1987).
Since lateral tyre forces are reduced by longitudinal ones, stability may be reinforced also if the front wheels are overpowered or overbraked compared to the rear wheels.
Figure 3. External forces and their resultant acting on a vehicle (air- or waterborne ship above, and car below). Stability and spontaneous return to original direction, since the resultant force applies to the rear of centre of gravity. Sequence of events and direction of motion from left to right.
Superior front wheel cornering performance, on the other hand, may initialize spontaneous skidding and spin-outs. That has been confirmed in a considerable number of accident case descriptions. A pilot investigation of some hydroplaning fatality cases, revealed that the tyre tread pattern of the skidding and outspinning cars was much deeper at the front than at the rear (table 1).
In the year of 1986, 327 car drivers and 155 passengers
were killed in Swedish traffic accidents, that is 9 car
occupant fatalities per week. According to table 1, rear wheel skids and poor rear tyres may contribute about a quarter of this average during certain time periods. However, a more scientific and systematic approach is needed before any general conclusions can be drawn on the relative in uence from the front tyres compared to the rear ones. Therefore, a case-control investigation is being planned to study if poor rear tyres are overrepresented in accidents on slippery road surfaces. See the last paragraph of this paper.
Table 1. A sample of car occupant fatalities reported as hydroplaning accidents, where it has been attempted to assess the tyre tread pattern depth. Cases (2 8) discovered in Swedish newspapers from four summer weeks in 1988, and an additional accident (Acc. ref. no. 1) from 1987, the documents of which were investigated on request from a local Court.
TYRE PATTERN (um)
Rear Front Front Comments
Right Left Right
ACC.
No. of DEVIATING CAR
Ref.
Date
Killed Skid Wheels Rear
No.
People
at
Driven Left
1 July 25, 1987
3
Rear Front
2?
2 July 25, 1988
1
Rear Rear
0-1
3 July 29, 1988
1
Rear Front
0 1
4 July 29, 1988 (1) Front Rear 2.5
5 July 30, 1988
1
Rear Rear
Tyres
6 Aug. 4, 1988 3 Rear Rear 0.5 1
7 Aug. 15, 1988 1 Rear Rear 2
8 Aug. 20, 1988 2 Rear Rear 1
No.2 8:
10
No 2 8 but no.4: 9
No.1 8 but no.4: 12
< 1.6
New
New
BIN 794. Rear tyre photos only.
Burned > 6
> 6
KY" 965. Car on fire.
2
4
4
nm=99L
2.5
3
3
BZ.) 434. Plow out (Front skid).
burned & destroyed Km 343. l linibus skidding.
1
4 5
4 5
EBK 322.
2
7
7
NZD 964. Single vehicle acc.
1 3 4 3 4 TSUBB54.
Hydroplaning fatalities in 4 weeks discovered in this survey.
Rear wheel skid fatalities in 4 weeks discovered in survey.
Rear wheel skid fatalities. Total number discovered in survey.
Stability and Controllability in
Different Situations and Cars
Directional (yaw) stability is often reduced when controllability is improved. Therefore, some measures on the car may deteriorate safety, though they are based on many individuals experience from normal driving and though they aim at increasing the driver s control over the vehicle.
Also contributing to the contradiction is that skilled drivers are able to better control the motion of the car by forcing it into an unstable state. For instance, in competition driving or the like it may be advantageous to turn the car quickly and precisely. Then the throttle may be used to spin the wheels and the hand controlled (parking) brake to lock them up.
This was demonstrated in a television program (Billing, 1988), where the VTI test driver kept the car spinning in about nine revolutions while its speed decreased from 70 km/h to standstill. The car was front wheel driven and the hand brake acted on the rear wheels.
In the same broadcast it was made clear that severe yaw motions, being extremely dangerous in real traffic, may occur unintentionally and suddenly if the rear wheels are overpowered or overbraked. Such cases may well be initialized on icy roads by the automatic cruise control devices, though they are considered contributing to safety by helping the driver to keep the speed limits.
It has also been observed (Strandberg, 1988) that rear
skid recovering may be simplified by braking in certain ABS-equipped cars. Since overbraking of the front wheels reduces their side forces, the need of precise countersteering becomes less pronounced.
Front Wheel Drive and Studded Tyres
Common driving experience indicates that severe yaw and rear wheel skids are unlikely to occur in front wheel
driven cars. However, irrespecitve of front or rear drive,
Strandberg (1988) found spin outs more frequent than plow outs in double lane change tests on ice with ordinary stud ded tyres. Since then it has been observed that the stud protrusion tends to increase at the driven wheels and to decrease at the other ones, particularly in front wheel driven cars. Therefore, front wheel driven cars with studded tyres may be more susceptible to unexpected loss of stability on winter roads.
Stud protrusion was measured by the VTI during the winter 1988/89 on a sample of 200 cars in Sweden. To achieve results representative for the Swedish tyre popula
tion, police officers selected the cars randomly from the
normal traffic ow on suitable roads. The average stud protrusion was greater at the driven wheels. The difference was most pronounced and statistically significant for front wheel driven cars. See table 2, confirming that front wheel driven cars with studded tyres tend to be less stable on ice, when longitudinal tyre forces are negligible.
The corresponding hazard of front wheel drive is rein forced by the fact that common education recommends drivers (even of front driven cars) to depress the clutch pedal before countersteering in a rear wheel skid. The rec-ommendation may stem from successful tests with experi enced drivers, who are unlikely to resist countersteering until the skid is fully developed and to avoid any recovering action before depressing the clutch pedal. See Strandberg
(1988) for further details.
Nevertheless, studded tyres on today s average car seem to improve safety on ice and snow according to the prelimi-nary results from a case-control (pilot) study carried out in two Swedish police jurisdictions from November 1988 to 827
Table 2. Stud-protrusion-statistics. Data from Random selection of 200 cars in traffic on Swedish roads during winter 1988 89. (Report by Samuelsson in preparation.)
Driven
Average Tyre Stud
No.
Variable Std.
Std.
Min.
Max.
wheels Protrusion of Mean Dev. Error value value Comments
(Variable Definition) Cars (mm) (mm) (mm) (mm) (mm)
Rear Rear Right 90 0.89 0.46 0.00 2.00
Rear Rear Left 90 0.90 0.47 0.00 1.90
Rear
Front Right
86
0.84
0.44
0.00
1.70
Rear Front Left 86 0.83 0.46 0.00 1.90
Front Rear Right 72 0.75 0.50 0.00 2.30
Front Rear Left 71 0.80 0.50 0.00 2.30
Front Front Right 75 1.01 0.51 0.10 2.60
Front Front Left 75 0.96 0.50 0.00 2.60
Rear Rear minus Front 86 +0.06 0.34 0.04 0.95 +1.05
Front
Rear minus Front
71
-0.21
0.27
0.03
1.15 +0.45
Mean signif. < 0
April 1989 (Strandberg & Junghard, in preparation). Based on the share of studded tyres in 30 accident involved cars and in 126 cars bypassing the accident site, the accident risk was assessed 90% greater for cars without studs. However, the safety superiority on ice with studded tyres was not significant (0.1 < p < 0.2), and the relative risk is expected to
be different (from 1.9) when more data are available for
analysis.
References
Aschenbrenner M, Biehl B, Wurm G (1988): Mehr
Vehrkehrssicherheit durch bessere Technik? Feld-untersuchungen zur Risikokompensation am Beispiel des
Antiblockiersystems (ABS). Unpublished, Mannheim,
1988.
Billing P (1988): Slipperiness. Feature of the Channel 1 program Science . (In Swedish: Halka. Inslag i Kanal
l programmet Vetenskap .) Swedish Television
Broadcasting Company, Stockholm, January 30, 1988. Glad A (1988): Phase 2 in Driver Education. Effects on Accident Risk. (In Norwegian: Fase 2 i Föreropplaeringen. Effekt på ulykkesrisikoen.) Report no. 0015/1988,
Transportokonomisk Institutt, Oslo.
Rompe K, Schindler A, Wallrich M (1987): Advantages
828
of an Anti Wheel Lock System (ABS) for the Average Driver in Difficult Driving Situations. Proceedings (pp.
442 448) on the 11th ESV Conference, Washington DC,
May 12 15, 1987.
SAE Society of Automotive Engineers (1975): Vehicle Dynamics Terminology. SAE J670D, Warrendale, PA 15096.
SCB Statistiska Centralbyrån (1986): Traffic Injuries 1985. Official Statistics of Sweden S 1 15 81 Stockholm. SCB Statistiska Centralbyrån (1987): Traffic Injuries
1986. Official Statistics of Sweden, S l 15 81 Stockholm.
Strandberg L (1983): Danger, Rear Wheel Steering. Journal of Occupational Accidents, vol. 5, pp. 39 58. Elsevier Scientific Publishing Company, Amsterdam.
Strandberg L (1988): On Skidding Accidents and Skidpad Training. Theoretical Analysis and Tests with Experienced Drivers. (In Swedish: Om sladdolyckor och halkutbildning. Teoretisk analys och körförsök med vana
forare.) Note TF 60 04, Swedish Road and Traffic Research
Institute, Linköping, October 24, 1988.
Strandberg, L, Tengstrand G, Lanshammar H (1983):
Accident Hazards of Rear Wheel Steered Vehicles. In Johannsen G & Rijnsdorp J E (eds.): IFAC Analysis, Design and Evaluation of Man Machine Systems, pp. 399 405, Pergamon Press, Oxford and New York.
