Conspicuity Of PTWs

Conspicuity of PTWs

20th. Nov 2000


It is clear that PTWs are involved in a disproportionate number of accidents where other vehicle operators fail to see them coming and turn into their path. This has serious consequences for the PTW user due to the poor safety performance of current PTWs.

I have been involved in developing better PTWs, seeking to improve comfort, handling, economy and safety, for twenty five years. In the course of this development I have become aware of several factors, which influence conspicuity, that I believe will be of interest to investigators of this subject. I have also considered nightime conspicuity and propose a specific experiment. I refer in the text to vehicles shown on the front page of my website.

Physiological factors relating to conspicuity.

During the Eighties research was done in England into the high incidence of bird collisions on motorways. It was felt that bird vision was being fooled in some way by vehicles. This research studied how birds judge distance, given their limited stereoscopic vision.

It was seen that a ‘one-eyed’ technique for judging distance, ‘movement against background’ might not work if an object was approaching on a steady bearing, with no movement against background. Objects approaching on a steady bearing are necessarily on a collision course so this is clearly significant.

Another one-eyed technique, ‘apparent increase in size’ was also seen to be ineffective when the object is too big for the edges to be seen, or so narrow that the apparent size increase happens too late for avoidance to succeed.

Both these techniques rely heavily on seeing the edges of any oncoming object. Eyes are ‘wired’ to specifically recognise edges and corners, emphasising the evolutionary importance of these techniques. If an object has indistinct or irregular edges, a ‘fuzzy’ outline, both techniques are hindered.

The Human case.

Humans rely heavily on their superior stereoscopic vision which allows their brain to make a direct ranging assessment which is normally quite accurate enough to avoid motor vehicles.
As a result drivers confidently rely on vision to deal with traffic.

However in the classic PTW accident the other driver will be looking to one side for approaching traffic. One eye is obscured by the nose and, unconsciously, the driver will revert to assessing ‘movement against background’ and ‘apparent increase in size’ to judge the position of approaching traffic. The brain will be looking for edges and corners to separate objects from the background.

The PTW conspicuity deficit.

In the above context it is clear why PTWs have a problem. The traffic stream consists very largely of cars, vans and LGVs. All have clear outlines which consist almost entirely of edges and corners. It is common for them to be brightly coloured and to stand out sharply against a background. These visual aids make judging their position and speed relatively easy, even using the ‘one eye’ techniques referred to above.

A typical PTW looks very different. The outline is typically cluttered and colour panels, if any, are small and highly variegated. Some paint schemes almost amount to ‘dazzle camouflage’ Most significantly PTWs are very narrow compared to all other vehicles.

These features mitigate directly against one-eyed detection. The typical PTW is difficult to separate from the (especially urban) background due to it’s unclear outline. If on a collision course ‘movement against background’ will be minimal or absent and narrowness will ensure that ‘apparent increase in size’ will be ineffective until the PTW is far too close. The result appears to be that the information relating to the dimly perceived and ranged PTW is simply not processed. Post-crash comments can be summed up as “It suddenly appeared out of nowhere!”

Improving Conspicuity.

I am certain that providing a PTW with a clear outline and filling that outline with a bright colour like white, yellow or orange, will assist other vehicle users in separating the object from the background and correctly judging it’s distance. It will be seen on my website that we have adopted this approach and the yellow (prototype) Voyager is regularly assumed to be an oncoming car. It is in fact 660mm wide.

Although it is not applicable to other PTWs I also note that the head fairings I have utilised on all the Voyagers and some other prototypes, usually painted the same bright colours as the rest of the vehicle, provide excellent conspicuity, being visible through other vehicle windows.

At it’s simplest, PTW conspicuity will be improved if they look more like other vehicles with clear outlines and solid panels of bright colour.

Active techniques.

In addition to the above ‘passive’ conspicuity features I have also developed driver techniques which I believe dramatically improve the conspicuity of a PTW. They are based on the above understanding of the problem of ‘one-eyed’ vision.

Operator technique (‘other vehicle’ driver) 1.

This is copied from chickens. When they wish to judge the distance of an unmoving object they move their heads back and forth. This produces background movement relative to a closer object and provides a way to judge depth. Anyone can try this by looking at a distant object with one eye and moving their head. Depth perception is improved. This is a useful technique for drivers at junctions.

Operator technique 2.

A PTW driver can consciously move off collision courses with a vehicle. This can amount to a gentle ‘slalom’ when approaching vehicles waiting at junctions etc. My experience of using this technique, watching drivers suddenly ‘see’ my oncoming PTW, is so convincing that I have been recommending this technique for some years.

PTW Conspicuity (daylight) conclusion.

I believe the narrowness and often cluttered outline of PTWs makes then inherently difficult to see and the unnoticed loss of stereo vision by other road users, when looking sideways, increases this problem.

Clearing up that outline, and using bright colours helps conspicuity. Education of vehicle users generally of the limitations of vision and PTW operators specifically about enhancing their conspicuity by riding techniques might also be expected to provide a very cost-effective reduction in this type of accident.

Nightime conspicuity.

In addition to the case outlined above there is the matter of nightime accidents where PTWs, running headlights on empty, dark roads, have not been seen by other road users. In this case only the light/s are visible and it follows that there is a problem for other vehicle operators in correctly scaling and judging the distance of oncoming PTW lights.

I am certain that this problem has been magnified by the recent European directive 93/92 EEC which allows PTW headlights the option of being up to 200mm apart, instead of the English vehicle lighting regulations which required them to be ‘immediately adjacent’. The consensus on this subject in specialist magazines and discussion groups being that it was a very unfortunate development!

However, even immediately adjacent headlights may be overlooked or incorrectly assessed. The problem appears to relate to the lack of any scaling tools. Car, vans and LGVs all have widely separated lights which can be seen at a great distance and easily judged. Approaching PTW lights either look like a far off car (93/92 EEC) or are seen a single point of light which is almost impossible to assess in distance terms.

Nightime solutions.

Making the light pattern of PTWs useable as a scaling tool, like those of other vehicles, would seem to be a way forwards.

Vertically separated lights, the opposite of the horizontal separation found elsewhere, might help, although there may be problems achieving sufficient separation to work at a distance.

Ground illumination is another possibility. This might consist of a relatively low power, diffused light, directed at the ground around the front wheel of the PTW. This would generate a pool of light, separated from the main light, that would move about as the PTW steered. This could provide the scaling and distance judgement needed by observers of oncoming PTWs.

Both techniques would identify the vehicle as a PTW, which might help other drivers to assess its speed.

Conspicuity experiment.

These ideas could be tested using simulation. It would be straightforward to incorporate various light patterns, including ground illumination, in a ‘night driving’ simulator program so that the ability of a large number of subject to assess the distance and speed of oncoming light patterns could be measured. This would allow a rapid and risk-free judgement of these possible solutions. I have proposed such an experiment to the British DETR and a vehicle lighting manufacturer but I am not aware that any action was taken.

Additional comment.

While I agree that PTW conspicuity is important because of their narrowness I would like to note that the serious consequences of vehicles turning into the path of PTWs are largely the result of the poor safety performance of PTWs. While such small vehicles will always be limited in the protection they can offer, the majority of accidents of this type take place, in England at least, at speeds below 30 mph in urban areas. Improving the safety performance of PTWs, by improved design, should usefully reduce these deaths and injuries.

Copyright Royce Creasey November 2000.