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by Matt de Neef
July 16, 2020
Photography by Adrian Williams/Unsplash
Any cyclist who’s spent time riding on the road will know the horror of a close pass. That terrifying rush of wind, the car mirror that seems to pass mere inches from your elbow, the driver either oblivious of their actions or, worse, entirely aware of the distance they haven’t left.
For decades now researchers around the world have been investigating how much space drivers give when overtaking cyclists, and the factors that affect that amount of space. Indeed overtaking distance is often used by researchers as a surrogate for cycling safety — while a close pass doesn’t mean a crash is going to occur, it does increase the chances.
A couple months back, a PhD student at the Queensland University of Technology, Elisabeth Rubie, led a team that compiled a review of the known literature when it comes to overtaking distance. Published in the Transport Reviews journal, the paper is an interesting and valuable look at the factors that influence how much space drivers leave when passing cyclists.
After filtering through an initial selection of dozens of papers, Rubie found 42 articles that she deemed eligible for inclusion, representing a total of 36 independent studies. All papers were either published in peer-reviewed journals, were US Transportation Research Board Annual Meeting papers, or were from government reports.
Before we dive into what Rubie found, it’s worth introducing a concept that she returns to several times in the course of her review: the so-called ‘Comfort Zone Model’. This model can be used to understand the decisions drivers make when overtaking cyclists. In the words of Finnish researcher Heikki Summala “drivers attempt to adhere to safety margins (comfort zones) of time-to-collision and distances between their vehicle and other road users. Drivers feel discomfort when other road users cross the threshold of these comfort zones, triggering them to change their behaviour.”
We can understand overtaking distance as an expression of a driver’s comfort zone boundary to the cyclist. The Comfort Zone Model predicts that, in Rubie’s words, “infrastructure, legislative, vehicle, driver and cyclist factors will influence a driver’s comfort zone to the cyclist and hence, [overtaking distance].” With that in mind, let’s dive into what the review found.
Image: Max Bender/Unsplash
Rubie considers a lot of factors in her review, some of which seem to have a greater impact on passing distance than others. What follows is a look at some of the most relevant factors. For the complete list, be sure to check out the full paper.
A total of 17 papers in the review considered the influence of road lane and shoulder width on passing distance. Fourteen of those 17 studies reported a significant increase in overtaking distance as lane width increased. So from a cyclist’s perspective, the wider the lane the better.
Fourteen papers in the review considered whether the presence of a bike lane affected overtaking distance. Ten of those 14 reported either no significant difference or that bike lanes were actually associated with less space. One of those papers came from Dr Ben Beck of Melbourne’s Monash University, and is one we’ve discussed on CyclingTips in the past.
It isn’t all that surprising that bike lanes alone don’t lead to greater overtaking distance. As researchers have long suggested, it might be that the presence of unprotected bike lanes gives drivers the impression that the designated area is all the space cyclists need and that, as a result, they come closer than they would otherwise. It’s a different story when the lanes are more protected, though.
American research from 2018 found that overtaking distance tends to increase as bike lanes get more protected. Which makes sense, as Rubie explains: “This is consistent with the [Comfort Zone Model] as hitting a bollard would damage a vehicle, resulting in a larger comfort zone for the driver. Alternatively, drivers may find it easier to judge the distance to bollards, and thus leave a greater distance.”
Image: Rodrigo Gonzalez/Unsplash
Sharrows are road markings that indicate that a lane should be shared with cyclists and where, ideally, cyclists should ride. Four of the five papers that investigated the impact of sharrows found no increase in overtaking distance for roads that had sharrows versus those that did not. As with painted bike lanes, this isn’t terribly surprising — it seems to be easy for drivers to assume that the designated area is all that cyclists need.
Poor road surfaces
A single study out of Taiwan considered whether road surfaces affect overtaking distance. That study found that poor road surfaces led to a 17% smaller overtaking distance than good surfaces. As Rubie suggests, this might be the result of cyclists and drivers being forced together at times to avoid road surface flaws like potholes.
Of five papers that looked at the influence of traffic density, three found that high-density traffic was linked with a smaller overtaking distance. This is perhaps unsurprising — more traffic means less room for cars to maneuver around cyclists, particularly in the case of oncoming vehicles.
Image: Eduardo Enrietti/Unsplash
A total of 15 papers suggest that the presence of an oncoming vehicle or a vehicle in an adjacent lane was linked with a smaller overtaking distance. This makes sense in the context of the Comfort Zone Model — drivers know that a collision with an oncoming or adjacent vehicle is likely to do considerable damage to their vehicle, and so the distance to the cyclist is often what has to give.
The influence of speed limits was examined in nine papers, with five of those reporting that higher speed limits are linked with greater overtaking distances.
Rider distance from the kerb
Seven papers found that the closer the cyclist rode to the kerb, the more distance drivers tended to give them. This is perhaps somewhat counterintuitive — riding closer to the kerb can give the impression there’s enough room for a driver to overtake in the same lane, often leading to a closer pass than might otherwise occur. Cycling advocates recommend riding further out into the lane, to encourage overtaking drivers to move into the adjacent lane rather than trying to squeeze past.
Not only that but, as Rubie notes, “close cycling to the kerb can also be problematic as there is more likely to be debris, glass, potholes, drainage covers or other dangerous impediments for the cyclist.”
The influence of gender on overtaking distance was examined in seven papers, but no consensus was reached. Three observed that drivers gave more room to cyclists that appeared to be female (including British researcher Dr Ian Walker wearing a wig in one study). One paper out of Queensland found that gender had no influence on overtaking distance, while research out of Minnesota found that female cyclists were afforded 7.6 cm less room than male cyclists.
Four papers considered whether a cyclist’s clothing — street clothes versus cycling-specific gear — made a difference to overtaking distance. In two studies, street clothes were linked with greater overtaking distance, perhaps because of the assumption that riders in street clothes are less proficient and therefore require more space.
Image: Brett Jordan/Unsplash
Two studies found that higher cyclist speeds were linked with a smaller overtaking distance. As Rubie notes, this might be because “drivers perceive the speed of the bicycle as indicative of cyclist ability, therefore a faster cyclist is perceived as more skilled and less vulnerable. These perceptions lead to a prediction of smaller comfort zones (therefore [overtaking distance]) when passing faster cyclists.”
But this finding was far from universal — another two studies found that higher bike speed was related to a larger overtaking distance.
Cyclist group size
A Spanish study from 2019 looked at overtaking distance for solo cyclists versus different formations of groups of up to three riders. It found that three riders in a straight line were given the largest average distance by drivers, followed by the single cyclist. Two riders abreast were given the smallest average overtaking distance, with close passes (defined in Spain as up to and including 1.5 metres) rising from 5% in the case of single-file riders, to more than 15% for those two abreast.
Research on the influence of helmet use seems to be inconclusive, with different studies — and even different investigations of the same data — offering mixed interpretations of whether helmet use is correlated with more overtaking space.
Image: Micheile Henderson/Unsplash
A total of 14 papers looked at the influence of vehicle type and size with most suggesting that the bigger the vehicle, the lower the average overtaking distance. Buses seem to be the worst offenders, with five papers showing that buses tended to give significantly less room than cars. Motorbikes, though, seem to give the most room, at least according to a 2014 study from Michigan.
This all makes sense — assuming a driver is trying to stay in the lane while overtaking, the larger the vehicle, the less space left as a buffer for the cyclist. Motorcyclists, however, can pass easily within the same lane while offering plenty of overtaking distance.
Of seven papers that considered the influence of motor vehicle speed, five found a significant increase in overtaking distance as speed increases. As Rubie notes, “the [Comfort Zone Model] predicts that driver comfort zones will be larger at higher motor vehicle speeds which the majority of these studies found.”
Five papers in the sample considered the influence of the driver’s passing strategy: whether they slowed then accelerated past, or whether they flew past without slowing. The findings here were inconclusive: a Swedish study found that accelerative passes were related to larger overtaking distance — drivers waiting behind the cyclist until it was safe to overtake with plenty of room — while a 2019 simulator study showed that flying passes were associated with larger overtaking distances.
Image: Robert Iana/Unsplash
As you can see, there are few conclusive answers to be found in the literature. But that’s probably to be expected. These studies come from different research groups in different locations around the world. Different jurisdictions have different laws, different road infrastructure, different attitudes to cycling, different traffic volumes and different numbers of cyclists. That’s a lot of variables at play and as Rubie notes, those factors all intertwine, making it difficult to assess any objective, universal truth.
“The influences of road width, number of lanes, lane width and presence or absence of bicycle lanes are difficult to disentangle in real-world studies and this may not be possible even in experimental studies,” she writes. “The interplay of these variables may well have contributed to inconsistent findings from studies which varied in the range of factors that they measured.”
Even the same study conducted in different parts of the same state can often return different results, as Queensland researchers found out when they looked at the impact of minimum passing distance laws a few years ago. And that’s not to mention the fact that different study groups use different methodologies, they have different ways of measuring passing distance, and are focused on different things. Again, it’s hardly surprising there are few absolutes to be found.
Image: Coen van den Broek/Unsplash
It’s also worth mentioning the limitations of this review. As Rubie notes, the fact that only English-language papers were considered narrows the focus of this review. There’s also a noticeable dearth of papers from developing countries. Given the importance of the bicycle in such nations, this lack of research — both in this review and more generally — creates a gap worth filling.
With all that said, here’s what we can say with some certainty from Rubie’s work. We know that a driver’s comfort zone to a cyclist is “highly influenced by the amount of road space available”. Lane width and shoulder width impose constraints on whether and how safely a driver can overtake. The presence of oncoming or adjacent vehicles further constricts the available space. Less space for drivers seems to result in less space between those vehicles and cyclists they are passing. In essence, the wider the lane, the better it seems to be for cyclists.
In a similar vein, rider safety seems to be most compromised by larger vehicles due to a lack of space. Buses appear to be particularly problematic in this regard.
And then there’s the issue of bike lanes. Paint on the road doesn’t seem to offer any protection to riders, but protected bike lanes do. Indeed researchers have long known that creating physical separation is a clear route to increased safety and this review only seems to emphasise that point.
Although there seems to be little consensus on the factors that lead to close passes, cyclists know one when they see one. Until improvements in the built environment and driver behaviour can reduce their prevalence, though, perhaps there’s some comfort to be found in the fact that researchers the world over are trying their hardest to improve the situation for everyone who rides a bike on the road.