Is hi-viz clothing harmful?

This article was published in 2018, in Newsletter 140.

Tin et al. (2015). The role of conspicuity in preventing bicycle crashes involving a motor vehicle.

The authors surveyed 2,438 people who took part in the 2006 Lake Taupo Cycle Challenge in New Zealand and combined that with follow-up surveys over the following six years, connecting their data to police records of highway crashes. They divided the respondents up into four categories of conspicuity-garb usage based on the answers to the surveys:

• Class 1: ‘usually conspicuous, by day and night’.
• Class 2: ‘often conspicuous by day, does not cycle at night’.
• Class 3: ‘occasionally conspicuous, by day and night’.
• Class 4: ‘rarely conspicuous during the day, but conspicuous at night’.

Among other results, they found that the ‘usually conspicuous’ Class 1 riders were most likely to be involved in a crash with a motor vehicle, at an average rate of one crash every 73 person-years. The people least likely to be involved in such a crash were the ‘occasionally conspicuous’ Class 3 riders, at an average rate of one crash every 146 person- years. The most dangerous part of New Zealand was found to be Auckland, where Class 1 riders experienced a near doubling of risk, tempered by a slight reduction in risk for Class 3 riders, but in general there were many fewer cyclists in Auckland than other parts of NZ.

Verdict: wearing hi-viz was correlated with higher risk of crashes, especially in Auckland.

Rogé et al. (2018). Does a yellow jacket enhance cyclists’ sensory conspicuity for car drivers during daylight hours?

The authors conducted experiments using a fully immersive urban driving simulator built within the cabin of a Peugeot 208. LCD screens were mounted over every window. Participants were asked to drive through a diverse set of road situations, in daylight conditions, with a wide variety of scenery and other road users introduced in a randomised but controlled order.

Each driver was tested by many different encounters with cyclists, who were displayed in the simulation wearing either grey street clothing or a conspicuous yellow jacket. The scenarios were broadly categorised as ‘high cyclist visibility scenario’ in which the cyclist was positioned almost directly in front of the driver, and ‘low cyclist visibility scenario’ in which the cyclist might be off to the side or out of the direct line-of-sight of the driver.

Drivers were asked to record the moment that they noticed the cyclist in a given scenario and then rate how easy or hard they thought it was to see the cyclist. The simulation recorded two objective measures: the distance of the cyclist when first noticed and if the driver then crashed into that cyclist.

The results found that drivers hit yellow-jacketed cyclists at a slightly higher rate, regardless of scenario, although the difference was not statistically significant. Similarly, when participants rated their ease of detecting the cyclist, there was no significant difference based on the colour of their clothing. Clothing was only a significant factor in one set of situations: for the ‘high cyclist visibility scenarios’ the drivers were able to perceive the yellow-jacketed cyclist at a longer distance (while still hitting them at the same rate). However, the yellow jacket was not helpful in the ‘low cyclist visibility scenarios’, which are considered to be the scenarios where help was needed most.

The authors surmise that one reason the yellow jacket is pointless has to do with the fact that the human eye has poor colour perception in peripheral vision.

Verdict: hi-viz made no difference in avoiding crashes.

Miller et al. (2017). Use of conspicuity aids by cyclists and risk of crashes involving other road users.

The authors looked at cycling-crash cases from Nottingham University Hospitals and for each one contacted the patient for an interview to understand more about the person’s riding style and personal choices about conspicuous clothing at the time of the crash. After successful interviews they then went out to the site of each of those crashes and surveyed random people who were found to be cycling there in a similar manner to the patient, in order to gather ‘control cases’ for comparison purposes. Surveys included questions about colour of clothing, helmet usage, type of riding, cycling experience, and various demographic and personal profile characteristics. Some interesting results were discovered:

• ‘Riding a racing or mountain bike rather than a commuter or folding bike was associated with a significantly increased crash risk.’
• ‘Cycling during overcast conditions or at dawn or dusk was associated with a significantly lower crash risk compared with daylight.’
• ‘[Persons injured] were more likely to live in a deprived area.’
• People with rule-breaking personality types were associated with having slightly fewer crashes.
• After filtering out other factors it was found that ‘there was an increased odds of a collision crash associated with use of conspicuity aids.

The authors suggest that these results may be due to a ‘risk overcompensation’ effect at work. In short, either people who wear conspicuity-garb may take more risks on the road or drivers may make unsafe assumptions and take more risks around people wearing conspicuity-garb.

Verdict: hi-viz may be harmful owing to the ‘risk compensation’ effect and the use of hi-viz is significantly associated with higher risk of crashes in Nottingham.

Prati (2018). The effect of an Italian nationwide mandatory visibility aids law for cyclists.

In October 2010, Italy passed a law requiring that cyclists wear ‘high visibility’ clothing when riding after dusk and before dawn, in addition to cycle lights. The law specifically says, in rough English translation: ‘cyclists riding outside of centres of population between half an hour after sunset and half an hour before dawn, or riding in tunnels, must wear a retro-reflective high-visibility gilet or bretelle’.

The author studied road crash statistics from before and after the law was passed. She found that the law had no effect on the number nor on the proportion of bicycles involved in road crashes. She offers three possible explanations:

• in fact, ‘high visibility’ clothing is pointless and useless;
• Italian cyclists may have resisted the requirement en masse;
• or, any gains were lost when motorists drove more dangerously around cyclists whom they assumed would be ‘more visible’ (a.k.a. the ‘risk compensation’ effect).

Verdict: the mandatory hi-viz law in Italy is pointless.

The seven outfits tested by Walker et al. (2014); the casual outfit used either a woolly hat or a sports cap depending on the weather.

Walker et al. (2014). The influence of a bicycle commuter’s appearance on drivers’ overtaking proximities: An on-road test of cyclist stereotypes and high-visibility clothing in the UK.

This is a famous study in which the authors gathered data on a bicycle fitted with a sensor that recorded motorist passing proximities. Over the course of six months the rider repeatedly followed a 26 km route in the southeast of England while wearing one of seven specifically designed outfits, chosen randomly each day. These outfits (shown above) were identified as follows:

• COMMUTE: a plain light blue cycling jersey
• CASUAL: regular clothing and a hat
• HI-VIZ: bright yellow cycling jacket
• RACER: racing-style red cycling jersey and lycra tights
• NOVICE: yellow reflective vest with words ‘Novice Cyclist Pass Slowly’ on back
• POLICE: yellow reflective vest with words ‘POLICE-witness.com – Move Over – Camera Cyclist’ on back.
• POLITE: yellow reflective vest with words ‘POLITE Notice Please Slow Down’ on back.

A total of 5,690 overtaking events were recorded, fairly evenly spread across the outfits. The average passing distance was 117.5cm. All the outfits were treated about the same by drivers, except the one labelled POLICE, which enjoyed a 4.5cm increase in average passing distance. Furthermore, no matter what the rider was wearing on any given day, approximately 1-2% of drivers came within 50cm of him.

Verdict: hi-viz was found to be useless against close passes.

Additional thoughts: arguably all of the outfits except the CASUAL one could be considered conspicuity-garb. Yet, the only significant change in behaviour observed was correlated with the one-letter difference between the words ‘polite’ and ‘police’. This suggests that conspicuity-garb served no role; drivers even had sufficient time to contemplate a word they read on the back of a vest, and change course accordingly.