18 Strategies – night-time conspicuity

SUMMARY – collisions between cars and motorcycles increase when it’s dark… drivers misjudge ‘time to collision’ more often when motorcycles have single headlights… retro-reflective material is often too high up or the approaching bike is at the wrong angle for the car’s lights to illuminate it… garments used at night feature blocks, stripes or even random patches of retro-reflective material and fail to create a recognisable shape…


It’s well-known that the collision rate involving motorcycles increases during the dark hours, and over the winter months in particular. So far I’ve talked about headlights in the DRL role, but the conventional single headlight of a motorcycle has been implicated in night-time collisions has been implicated in numerous studies, stretching back many years. It’s worth pointing out that at night, as in daytime, the motorcycle’s headlight has multiple roles. As well as lighting the road ahead of the rider, the headlight also has to function as a conspicuity aid:

  • at a range of distances at both urban and rural speeds
  • across a wide angle ahead of the motorcycle
  • in urban and rural environments
  • under a range of lighting conditions from brightly lit streets to unlit rural routes
  • under a range of background lighting conditions from multiple light sources to total darkness

The driver has to see the light, work out it belongs to a bike, then judge speed and distance. Once again, it seems unlikely that motorcycle headlights perform all these roles equally effectively. For example, in one recent study Gould et al (2012) found that:

“Results showed that individuals were significantly more accurate at judging the speed of two car headlights compared with the standard solo headlight motorcycle.”

Given the high crash rate at night, perhaps it’s not surprising that the thoughts of road safety campaigns turn to advising riders to make themselves more visible at night.

Advice often centres around the need to wear retro-reflective clothing. Here’s how a rider on a bike forum approached the problem some years ago. As well as sporting a standard ‘traffic vest’ with the pattern of reflective material around the chest and up over the shoulders:

“I have various bits of reflective tape stuck to my bike. I’ve got reflective tape round the bottom of the helmet, I wear a hi-viz vest with reflective tape on it and use the armbands that are given out free to schoolkids as ankle clips.”

That sounds like a pretty comprehensive head-to-toe strategy, doesn’t it? But does it actually work? Here’s a problem. As we’ve also seen in the prior discussion on DRLs, we’re asking reflective materials to perform several different roles:

  • conspicuity – to be seen in the first place
  • recognition – to be identified as ‘bike and rider’
  • judgement – to help the driver estimate speed and distance and ‘time to collision’

And we’ve already seen how a strategy that works in one role may not be effective in another.

Helmet halo
Taken from an advert for a ‘helmet halo’.

How do reflective materials work? The basic principles of light and reflection have been known about for hundreds of years. The first use of reflection for road safety purposes seems to have occurred as long ago as the 1920s when relatively large glass spheres (10 – 20mm in diameter) were attached to road signs to reflect the light from headlights and outline the sign’s symbols to the driver. The first reflective tape using tiny glass beads was made by 3M was developed later that decade and the first reflective road signs were trialled in the USA in 1939 and adopted in the UK in 1947 (Lloyd (2008)).

Modern reflective fabric and stickers use the same technique; the reflective nature comes from a coating consisting of a layer of tiny glass beads. The special property of what should be more properly called RETRO-reflective materials is that the beads bounce most of the light that falls on them directly back to the source of that light. In other words, retro-reflective materials do not reflect like a mirror, which bounces light off at an equal and opposite angle. Whichever way the retro-reflective garment is facing, and whichever direction the light comes from, as soon as light fall on the retro-reflective material, if the observer is standing in line with the light source, the garment will ‘light up’ thanks to the light being bounced back towards the source. With vehicle headlights on high (or main) beam, retro-reflective materials that are directly in the beam can be seen by the driver from hundreds of metres away. But…

…as soon as the angle between the light source and our eyes widens, the intensity of the reflection rapidly falls off. So this is the first issue with retro-reflective materials; if the wearer is NOT directly in the headlight beam, then no significant amount of light will bounce back to the driver. And that leads to a very practical problem in urban driving and unlit roads with oncoming traffic. Nearly all drivers will be using low beam.  Have a look at the photo. I’m wearing a conventional traffic vest with retro-reflective material around the upper back and up over the shoulders. I’m also wearing a riding suit which has two large patches of retro-reflective material on the back of the calves. The bike with the camera on is on dip beam. It’s that awkward twilight period after sunset but before full darkness. Which set of retro-reflective material shows up?

Effectiveness of retro-reflective clothing

I’ll hazard a guess you spotted the retro-reflective panels on the lower legs before you noticed that the hi-vis vest also had reflective stripes.

We already know that the urban SMIDSY collision involves:

  • a vehicle emerging from a side turning
  • an approaching vehicle turning right across the bike’s path

Think about the vehicle’s lights. Where are they likely to be pointed? In either case, the aim of low beam is down towards the surface and in the case of the emerging vehicle,they will be shining across the road ahead of the bike. In either case there will be no significant reflective effect from material on a hi-vis jacket.

Of course, neither the headlight focus nor the retro-reflective effect are 100% perfect, so even when not perfectly lined up there is always some ‘light scatter’ bounced back to the observer. And whenever there are other light sources, there is a bit of ‘light spill’ which reflects back to the driver. Put the two together and there is a ‘glow’ to the material and we’ll see in another article how we can exploit that. But it’s important to understand that the spectacular reflective effect – as seen in sales brochures or safety leaflets – ONLY occurs when high beam directly illuminates the retro-reflective material. In short, the combined hi-vis / retro-reflective traffic vests recommended to motorcyclists were never designed with that use in mind. They were intended for roadside use – perhaps by maintenance workers or police – to help them be seen from a distance on unlit roads.

Streetworker with reflective clothing

A second issue is that the retro-reflective material has to be where it CAN be illuminated. Too often, the rider is wearing a traffic vest where the reflective material on the torso is either blocked from the front by a tall screen, or from behind by a top box, a rucksack or even a passenger!

Even if reflective material is placed firstly where light can fall on it, and secondly the angles are right to bounce light back to a driver, we still have the problem of being recognised as ‘bike and rider’. If a hi-vis vest can blend into a similarly-coloured background, light from a retro-reflective garment won’t stand out when the background is brightly illuminated. And just as bright patches of colour can create disruptive camouflage, typically the garments used by motorcyclists at night feature blocks, stripes or even random patches of retro-reflective material.

It may be easy enough to identify an H belt or a traffic vest against a uniform dark background. But in a partially back-lit environment, the retro-reflective material may not stand out clearly.We may be able to spot the H pattern or a traffic vest when it’s clearly presented against a dark background, but against patches of dark and light, it may not be so easy to work out exactly what we’re seeing. And take a look at how manufacturers use reflective material on bike clothing – often as a logo. Here’s my own Two Zero branded waterproof overjacket. You can clearly see the reflective material. How does that help the driver recognise us as a motorcycle rider? I’ve no idea.

Waterproof jacket with hi viz

In the worst case, the brain’s sub-conscious data-processing filter, which works below the level of our consciousness to filter useful information which is routed to our conscious, attentive brain, makes no sense of the light / dark pattern. So although the light from the jacket arrives at our eyes, the alert that say “motorcycle ahead” and fires up the real-time, conscious and attentive part of our brain never gets sent. So we simply do not ‘see’ the motorcycle. The risk is that instead of helping conspicuity, these ‘creative’ retro-reflective patterns, and the mix-and-match strategy employed by the rider I mentioned at the top of the page, don’t guarantee that the driver will see us or understand what he’s seeing.

Although I’ve been talking about many of these issue since the very first SOBS presentation (and earlier online), it’s taken a while for these weaknesses to be accepted as likely problems. Many are mentioned in Helman et al (2012).

Do we have any evidence for retro-reflective clothing being effective in reducing SMIDSY collisions? If I find any I’ll tell you… or you tell me.

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Last updated:

Wednesday 1 May 2019 – minor edit for clarity

References:

Helman, S., Weare, A., Palmer, M. and Fernandez-Medina, K.(2012) “Literature review of interventions to improve the conspicuity of motorcyclists and help avoid ‘looked but failed to see’ accidents. Transportation Research Laboratory, Project report No. PPR638

Lloyd, J. (2008) “Understanding retroreflectivity – A brief history of retroreflective sign facesheet materials” www.rema.org.uk/pub/pdf/history-retroreflective-materials.pdf
Retrieved 14 December 2018

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Photo credit Paul Townsend
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