silhouette effects in night driving

SILHOUETTE E F F E C T S I N N I G H T DRIVING

GUNNAR JOHANSSON and KAm RUMAR

Department of Psychology, University of Uppsala, Sweden

JOHANSSON, G. & RUMAR, K. Silhouette effects in night driving. Scand. J. Psychol., 1971, 12, 80-89.-1n order to investigate within what areas on the road silhouettes, as framed by the light from a meeting car, can be of help to the driver, a series of experiments has been performed. The experiments employed two stationary opposing cars standing at given distances from each other, these distances (100-400 m) being determined by pre-experiments. On the area between the cars “iso-silhouette” curves were measured using a special method for a number of silhouette relevant conditions. Factors favour- ing silhouette effects were found to be: narrow road, short distance between the cars, high road surface reflectance, inner bend, the driver’s eyes being high above the road, particles in the atmosphere, large spread of the meeting headlight beams.

When driving at night on roads without fixed lighting a driver can detect objects on the road by means of two separate lighting effects.

I. Objects on the road are illuminated by the headlamps and any objects will thus stand out as being lighter than the darker background.

2. At a car meeting it is theoretically possible to detect objects also with the help of the meeting car’s lights. Here the object is seen as a dark silhouette against the illuminated road or atmosphere which lies between the object and meeting car, or it is detected because of its screening off one or both of the meeting car’s lights. In this report all such detections of obstacles with the aid of meeting lights will be referred to as silhouette effects.

There is a paucity of literature in this area, No systematic investigation into the increase of visible distance by means of silhouettes has been found in the literature. Roper (1939) presented curves showing visible distance with and without silhouette effects, but the report is very general in character and its aim was not the same as that of the present investigation. An investigation of Vermeulen & de Boer (1949) also contains some data on silhouettes.

Under what conditions can the silhouette effects during a car meeting give a driver information about obstacles in front of his own car but outside the range of his own headlamps?

The appearance of silhouette effects in a meeting situation (Fig. I) is primarily dependent upon the following three factors:

(I) The luminance of the background. (2) The size of the illuminated background’s visual angle. (3) The size of the object’s visual angle.

Factor (I) is dependent upon the reflection properties of the road, on the reflectance of particles in the atmosphere (dust, mist, rain, snow etc.), the light distribution of the meeting headlights, and on the distance. The background’s luminance is highly important since

80 Scamd. J . Psychol., Vol. 12,1971

SILHOUETTE EFFECTS IN NIGHT DRIVING 81

it determines the contrast relationship background/silhouette. The ability of the human eye in distinguishing a dark section from a lighter background is directly dependent upon the relation between the background’s luminance and the surface area of the darker stimu- lus. This relationship may be approximately represented by the simplified equation B x A = C (where B = brightness, A =retinal area, and C = a constant), the so-called Ricco’s laws in its basic form. For a more exact description of the relationship see Blackwell (1946).

FIG. I . Schematic illustration of the silhouette effect (see text).

Factor (2) is decided by the distance between the cars (a), by the height of the driver’s eyes above the road (h) and by the range of A’s light beams (b) (except during the short time in a dipped lights meeting when the object is inside A’s light beam and the distance from A to the object is applicable). These three part factors decide the visual angle (a) in the vertical plane.

Factor (3), the object’s visual angle in the vertical plane is determined by the angle a, for objects that are more than ca. 50 cm high. The visual angle in the horizontal plane is determined by the distance to the object, and of course the actual width of the object.

It follows that background +silhouette will be represented on the retina by a horizontal band of light with a dark break. The driver’s low height off the ground, (h), together with the limited range of the dipped lights on the meeting car’s left hand side (b) distance to opposing driver (u) mean that the band of light is extremely narrow (<0.5’).

The angle a is related to u, b, e, and h as follows:

bh tan a= h2 + (a - 6)

Fig. I viewed from a distance of ca. 25 cm gives a retinal image of the same size as that produced on the retina of a driver when the distance to the opposing car is IOO m and the distance to the object is 70 m. Angle a is then of the order of 0.3’.

Theoretical considerations, practical experience and the results of pre-experiments sug- gested that the following variables were of primary interest:

I . The road (a) reflectance of the surface, (b ) curvature (in both the horizontal and vertical plane), (c) width.

2. The position of the driver’s eyes. (a) in relation to the road’s centre line, (b) in height above the road.

3, Atmospheric conditions

4 . The meeting cur’s lighting system

Of these variables only I U , I C , and 4 were studied systematically during the experimental series which is reported here, although the others are also briefly discussed.

6 - 711948 Scand. J . Psychol., Vol. 12, 1971

82 GUNNAR JOHANSSON AND KARE RUMAR

METHOD

The road’s width was 7 m (this corresponding to the width of a normal Swedish two- lane highway) except in one experiment in which this parameter was investigated and a road width of 5 m was employed. All the investigations were carried out on straight, flat roads. An asphalt as well as a concrete road were used in one experiment. Only silhouettes of objects on the driver’s (subject’s) traffic lane were of interest.l

The cars’ lighting systems were in good condition. The dipped headlights were of the asymmetrical Continental European type.

The object which was to be silhouetted was the size of a human figure as viewed frontally (1.4 m high, 0.4 m broad). For all experiments the target was covered with dark grey cloth whose reflectance was 6 %.

The subjects were young and probably above average in the question of visual acuity (at least I, with glasses). As a rule three subjects were used in each experiment. They kept the same seats during the various conditions.

The experimenter held radio-contact with the subjects. He told them when to look away (the target being moved) and when to search for the silhouette (the target in position). Each presentation lasted 5 sec. Silhouette effects occur in an area of the road in front of the subject car and it is possible, in principle, to discuss them in terms of ”iso-silhouette“ curves for a given moment of the meeting.

I t was therefore decided that from the point of view of measurement it was appropriate to set several representative distances between the cars by pre-experiments and afterwards to plot “iso-silhouette” curves on the road area within these distances.

The standard series of distances between the cars which was chosen was zoo, 150 and IOO m. In the pre-experiments this series had been found to give adequate insight into the variation of the silhouette effect as a function of the distance between the cars.

The position for two iso-silhouette curves was set within each area of measurement. One (S,) represents a conventional threshold level method of limits for the three subjects who took part in the experiments. The other curve (S,) gives the limits at which the subjects were certain that they could see the silhouette.

The position for both these curves was fixed as follows: distances of 15 m from the subject car were marked off along the side of the road. At each of these intervals eight half- meter steps (0-3.5 m) were used as target positions (Fig. 2). The experimenter told the subjects to look down, trundled out the target to one of the eight half-meter steps and then told the subjects to look ahead. The subjects were to decide (independently of one another) if they could see the target as a silhouette. If they saw none they marked their protocols with a 0, if they saw a silhouette at the defined threshold level they marked with a I and if they were certain that they saw a silhouette they marked with a 2. After 5 sec the subjects were told to look down again, the target’s position was changed and the experiment continued as described.

The order in which the target was presented in the eight positions was determined previously with the aid of a random numbers table. When all eight positions at one target distance had been used, the target was moved to the next target distance and the procedure

This investigation was carried out in left hand traffic but since all relevant conditions are identical with right hand traffic the following is treated as right hand traffic.

Scand. 1. Psychol., Vol. 12,1971


repeated but with a different random ordering of the target positions. The experiment continued until the target was so near that it could be seen directly in the lights of the subjects' car-not in silhouette.

15 30 45 + 1 5 a - ~ 1 5 ~ 1 5 - + + etc.

4 " " " t l

FIG. 2. Outline of the experimental situation with the 8 target positions shown at 3 intervals. A is the opposing car and B is the subject car (m). The road is 7 m broad.

As a control of measurement precision the series was repeated, starting this time from the subject car and increasing it by 15 m each time. Thus, every condition was repeated twice.

The iso-silhouette curves S, and S2 were taken as the mean of all judgements for each target distance. When these points were connected up, S, and S2 were obtained.

INVESTIGATION OF ISO-SILHOUETTE CURVES

The reflectance of the road was expected to be a highly important factor for the appearance of silhouette effects. The reflectance can vary in two ways:

I . Roads have different surfaces according to the building material used. A reflectance series may be thought of as comprising light concrete through gravel, light asphalt, oil gravel to dark asphalt.

2. The reflectance can change temporarily with the weather, e.g. rain or snow.

Asphalt and concrete roads Pre-experiments showed that it would be sufficient to test the two extremes in the above

series of road surfaces. The investigation proper therefore used dark asphalt of 3% reflectance and concrete of 7% reflectance (reflectance 30 m in front of the car as seen from the driver).

The results of this experiment are given in Fig. 3 and can be summarized as follows: I . Under the given experimental conditions (straight road, and full attention on the part

of the subjects etc.) silhouette effects, as judged with certainty, only occur as a masking of the meeting car's lights with zoo m or more between the cars on both types of road. This may be interpreted as the silhouette's surface in terms of visual angle, not having passed the threshold of the human eye even at the level of luminance which the light concrete provides.

z. With 150 m meeting distance the dark asphalt still gives no sure silhouette other than in the form of masking (the curve follows the line headlamp-eye). However, the subjects began to report a silhouette at the threshold level. The light concrete differed from the

Scand. 1. Psychol., Vol. 12,1971

84 GUNNAR JOHANSSON AND d R E RUMAR

asphalt at this distance and silhouettes were seen. But even for concrete, the curve S, is about 2 m from the roadside, which means that the silhouette of a pedestrian walking 1.5 m from the roadside would not be noticed by a driver, under conditions similar to those used here.

0 15 30 45 60 75 100

concrete I s2 Oasphalto

I ................... " s1 0 ................. 0

0 15 30 45 60 75 90 105 120 135 150

.......... ................. ................... ........... ............ ............ El " *,;:,;..::" ~

D . 0 0 0

- - - - - . u . concrete . S 2 .asphalt

. s1 0. 0

0 15 30 25 60 70 90 105 120 135 150 165 180 200

3 FIG. 3. Means for S1 and S, (see text) on asphalt and concrete roads at three distances between the cars (m).

3. With IOO m meeting distance and target distance of over 30 m the two pairs of curves more or less coincide. The enlargement (in terms of visual angle) of the silhouette and its bright background has clearly such a large effect as to overshadow the reflectance difference between the two types of road. Safe silhouette effects (S,) do not occur here either until about 2 m from the roadside.

Neither condition gave an acceptable silhouette effect one meter from the roadside. The two conditions show a distinct difference only at about 150 m meeting distance.

Dry and wet roads An ideal diffusing surface reflects incident light equally in all directions. The grainy

structure of a road gives it this property to quite a large extent. Water on the road evens out the surface structure and, in the extreme case, water will effectively comprise the surface, which will then reflect light in the same way as a plane mirror. This means of course, that a water covered road gives a glaring light image of approaching headlights at a given moment during a meeting, while appearing otherwise completely dark. (A driver's own headlights are reflected away from him.)

Scantd. 1. Psychol., Val. 12, I97I

SILHOUETTE EFFECTS I N NIGHT DRIVING 85

From such elementary principles of optics it can be predicted that a wet, and more or less smooth, road surface will have a lower luminance when illuminated by approaching headlights than if it were dry. The following experiment was carried out in order to see how such a luminance change affects the silhouette effect.

The previously described method was used. It was considered sufficient to compare the roads with a distance of IOO m between the cars.

I w e t . S 2 dry

0 15 30 45 60 75 90 100 ’ * SI 0 Q

IAl FIG. 4. Means for S, and S, (see text) on dry and wet roads at one distance between the cars (m).

The lights on the subject car were not the same as those used earlier. They were control- led but did not give exactly the same light distribution on the road. The subjects were not the same in this experiment, either, and thus the absolute results of this experiment are not directly comparable with the earlier obtained results.

The road was concrete. When the part of the experiment dealing with the wet road was carried out there was no rain falling, but the road surface was wet and covered with small puddles. Otherwise, conditions and the three subjects were the same for the whole experiment.

The results are given in Fig. 4. The data show that a wet road surface does not give silhouette effects to the same extent

as a dry one. It is to be expected that this difference would be even greater when a less grainy road surface is considered instead of the landing strip used here. Also, the difference would increase if more water was covering the road than was the case here.

Another effect is that the target was seen in the subjects’ own lights at longer distances when the road was wet, this being due to the increased reflection of the headlights in the direction of the target.

Another well-known effect of this reflection is that a driver, in a situation with no meeting cars, may be said to loose contact with the road. The road is not seen when no light is reflected back towards the driver’s eyes.

WIDE AND NARROW ROADS

From the S, and S2 values obtained for a 7 m wide road it can be predicted that if the road was so much narrower that the roadside on the subject side was moved in 2 m, a large part of the S, and S2 curves would coincide with the side of the road. This means that the possibility of seeing silhouettes during a meeting should be greater on narrow road that on a wide one. This effect will naturally depend on the light distribution from the headlamps and can be read off their isolux curves. There is, however, one factor which could

Scand. J . Psychol., Vol. 12,1971

86 GUNNAR JOHANSSON AND K.kRl3 RUMAR

possibly offset this effect. When cars drive nearer the middle of the road the glare increases somewhat.

Glare during a dipped-headlights meeting is maximal when there is about IOO m between the vehicles whilst the luminance of the surface behind the target will be quite high even with a 7 m wide road. Thus, IOO m meeting distance will be the least favourable, from the

0 15 30 L5 60 100 " . " (31

'f- I31 lel

51 - s2 - 0 15 30 L5 60

"/

- - - A R G W - - -

0 15 L5 75 105 150

...................................................... ..a. ..... I71 .... 17) ,.. ..........

IA/ 51 .............. Sp-

0 15 L5 75 105 1 so ................ .................... I51

I 51 /------=\- .......

L2.A N A R R O W

FIG. 5. Means for S1 and S, (see text) on a wide (7 m) and on a narrow (5 m) roadattwodistances between the cars (m). The figures within brackets indicate the reduced number of observations on that interval (here normally 8).

point of view of silhouettes, for a narrow road as compared with a wide one. The longer this distance becomes, up to a certain limit, the better the narrow road will be. It was therefore decided to compare the roads using meeting distances of IOO m and 150 m. The widths of the roads were 7 m and 5 m and in both cases the cars were placed I m from their own near hand side of the road.

The road surface was dark asphalt and the visual procedure was carried out to determine S, and S, using 4 subjects this time. The subjects were not the same as those used in the previous experiments so the absolute values are again not directly comparable.

It may be seen from Fig. 5 that the narrow road allowed for seeing silhouettes nearer the roadside than the broad one. The effect is probably greater than it looks on the figures since no measurements were made outside the roadside.

Scasd. 1. Psychol., Vol. 13,1971

SILHOUETTE EFFECTS I N NIGHT DRIVING 87

FULL AND DIPPED HEADLIGHT

Full headlights provide a completely different distribution of the meeting car's light beams over the road than do dipped headlights. It is readily seen that the angle c( is greatly increased and that the luminance of the road will be correspondingly decreased. Arguing from Ricco's law it appears likely that these two effects would more or less cancel out but it seemed as well to test this experimentally.

full headlights dipped headlights s2 -

* S I O O

I-

O 30 1 5 €0 75 90 105 120 135 150 165 200

0

f u l l headlights .-. S1 and S2

135 165 195 225 255 285 315 LOO

FIG. 6. Means for S, and S, (see text) for dipped and full headlights at two distances between cars and for full headlights at one distance. The figures within brackets indicate the reduced number of observations on that interval (here normally 6).

The previously described method was used. However, with regard to the problem it was considered advisable to choose meeting distances of zoo, 200 and 400 m. Any superiority possessed by full headlights should show up primarily at longer distances. The silhouette effects with dipped headlights were not measured with the meeting distance 400 m since they were predictable from earlier results (S , = S, = the line where the target would block the light from A's headlamps). Also, only one measurement per subject for each figure distance was made with full headlights for this meeting distance, as the results were so unequivocal.

The results are given in Fig. 6. The assumption that full headlights would give better silhouette effects than dipped head-

lights when the meeting distance was long (zoo m or more) proved to be false. At these meeting distances, full headlights did not provide silhouettes until the target was practically

Scand. J. Psychol., VoL 12,1971

88 GUNNAR JOHANSSON AND KhRI! RUMAR

blocking the headlights of the opposing car. At shorter distances (100 m) full headlights proved to be the poorer from the point of view of silhouette effects. I t also follows from these results that for a meeting distance of 400 m S, and S, coincide.

FACTORS NOT SPECIALLY STUDIED Some of the variables relevant to silhouette effects mentioned earlier were not tested

systematically but the probable effects can be deduced from the results obtained, or from other observations made, during the experiments.

(a) Winter roads. It was noted during the course of the investigation that when the road was frosted over, the target was seen in silhouette as far out as the roadside at over IOO m distance. The explanation for this must be that the ice crystals reflect much more light towards the subjects than does a dry surface. Thus, when both the contrast between road and silhouette and the visual angle of the illuminated background increase the target will be perceptible as far out as the roadside. I t should be possible to generalise to winter roads. In this latter case, there are often walls of snow left at the sides of the road after snow- ploughs have been at work and these must also improve silhouette effects. On the other hand, if the roads have been heavily sanded and no snow-walls are present the conditions will be very similar to a summer road from the point of view of silhouette effects.

(b) Road bends. Some prediction is possible from the data obtained on straight roads. On a right hand bend (inner bend) the intensive central part of the opposing car’s light beams falls behind the target and a driver will have an excellent opportunity to see it in silhouette. Besides this, at some time during the meeting the target must stand directly in front of the opposing headlights, which will give the driver an unmistakable experience of a silhouette. In left hand traffic, on the other hand, a left hand bend will be more favourable to silhouette effects than a straight road.

When the driver has to negotiate a left hand bend proportionally less light from the opposing headlights will fall behind the target. Nor will the target be standing directly in front of the opposing headlights during any phase of the meeting. Thus, the possibility of seeing a silhouette will be less in this case than on a straight road, with possible exceptions when there are snow-walls or corresponding highly reflecting surfaces at the roadside.

When the meeting car drives downhill the angle a will increase, resulting in better silhouette seeing. On an uphill climb every possibility of seeing silhouette against the road will disappear. This should be amply compensated for, however, by the atmosphere behind the target being directly illuminated and functioning as a very favourable background surface for silhouettes.

(c) The height of the driver’s eyes above the road. Here, only a purely theoretical argument is offered. If the driver sits higher up in his car, glare will decrease and the visual angle (z) of the illuminated area behind the figure will increase in width. Both these factors will tend to give the driver more chance of seeing a silhouette. The reverse will occur if the driver sits lower down in his car, of course.

( d ) Atmospheric conditions. It was noticed that when particles were present in the atmosphere (e.g. dust, mist, snow or rain) which reflected the light from the opposing car behind the target in the direction of the subjects the light surface behind the target was considerably increased. This was accompanied by the silhouette becoming more conspicuous and

Scand. I. Psychol., Vol. 12, I g 7 I


the subjects detecting the target when still more than IOO m away from it, even if it was placed at the side of a broad road.

The reflection of light by the particles also means, that the driver’s visibility decreases, this of course being due to the “wall of light” caused by his own lights.

(e) The light distribution from the headlamps. It was noticed in a pre-experiment that a type of headlamp having a very concentrated beam afforded less chance of silhouette effects than another type having a more widely distributed light beam. I t was this latter type of headlamp which was used in all the experiments reported here. (f) Distance judgement. It was observed during the investigations that drivers had great

difficulty in judging their distance from the target when it was seen in silhouette at longer distances. For instance, it was not uncommon for a driver almost to hit the target because it was nearer than he thought.

The reason for this faulty distance judgement with silhouette could be the absence of a reference system. The driver sees that the “light band” is broken but does not know if it is broken by the top, middle or bottom of the target. Thus, he cannot decide whether the target is near at hand or far away.

CONCLUSIONS

The main conclusion from this investigation is that with our had headlights (Johansson & Rumar, 1968) silhouette effects reduce the number of accidents occurring in night driving car meetings, but that the effect due to its unreliability does not add considerably to safety. Drivers cannot increase their speed because they sometimes detect obstacles as silhouettes. The effect is too unpredictable to be of any real value. The relation between the brightness and the surface area of silhouettes generated by opposed headlights is such that silhouettes usually occur very near the visual threshold.

The fact that silhouette effects are normally very weak makes it probable that training is important in the development of the ability to pick up and use this kind of information.

It should be noted that these experiments were carried out with the headlight standard common in the midst of the 1960s. With the improved optical characteristics and more intense bulbs used in the 1970s the silhouette situation could be slightly different. The present headlight system is not constructed to give silhouette effects. I t is quite possible that some kind of “side-light” with the beams directed down towards the opposing lane could give the opposed driver substantial help in form of improved silhouette effects.

This investigation was made possible by a grant from the Official Swedish Council for Road Safety Research.

R E F E R E N C E S

BLACKWZLL, H. R. (1946). Contrast threshold of JOHANSSON, G. & RUMAR, K. (1968). Visible the human Eye. J . Opt. SOC. Am., 36, 624- distances and safe approach speeds for night 643. driving. Ergonomics, 11, 3 , 275-282.

VERMEULEN, D. & DE BOER, J. B. (1949). Inter- ROPER, V. & SCOTT, K. D. (1939). Silhouette national visibility tests with motor car light- seeing with motor car headlamps. Trans. ing. Mimeographed Report of the Zandvoort Tests to the C I E Committee, 23b.

Illum. Engn. SOC., nov., 1073-1084.

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S c a d J. PJychol., Val. 12,1971

silhouette effects in night driving

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