http://www.planetizen.com/node/50020
Beyond Safety in Numbers: Why Bike Friendly Cities are Safer
27 June 2011 - 9:00am
* Norman W. Garrick
* Wesley E. Marshall
Studies often show that bicyclists find "safety in numbers."
Norman Garrick and Wes Marshall explain that the street design
strategies that attract bike riders are the same ones that improve
road safety for all road users.
Davis, California. Photos by the author.
Davis, California, is widely celebrated as the bicycling capital of
the United States with over 16% of the population commuting to work on
bikes. What is less well known is the fact that the traffic fatality
rate in Davis is also unusually low, at about 1/10th of the California
statewide rate. Although this fact is not widely disseminated, there
is growing data showing that cities with very high use of bikes for
routine transportation almost always have much lower than average
traffic fatality rates.
The finding that most bike friendly cities are safer than average has
been reinforced by the recent experience of cities such as Cambridge,
MA, Portland, OR, and New York. These cities have garnered much press
for their success in dramatically increasing bike use over the last
several years. This increase in bike ridership has corresponded with
an equally dramatic decrease in traffic fatality rates in all three
cities.
Interestingly, the decrease in fatality occurred not just for people
on bikes, but for all classes of road users - including people in
cars and people on foot. In other words, the increase in bike use has
benefited all road users by helping transform the streets into safer
places.
So what is the cause of this beneficial relationship between bike use
and traffic safety in so many American cities? In the early 2000s,
Peter Jacobsen, one of the first researchers to report on the subject
of high biking cities being safer, suggested that the cause was
'safety in numbers'. He stated that high bike use cities were
generally safer than others because the very presence of bikers
conditioned drivers to behave with more care. Since then, the 'safety
in numbers' hypothesis has been routinely offered as the default
explanation for any improvement in safety associated with increased
bike use.
Bike lanes and track in Alameda, California.
While there is undoubtedly much truth to the idea of 'safety in
numbers,' this explanation by itself is incomplete and leaves many
questions unanswered. For one, it does not provide any guidance about
how to increase bike numbers to a point that will bring about
increased safety. Also, it founders on the classic chicken and egg
problem. Do high numbers of bikers bring about safety or does a
perception of safety bring out more bikers?
The key issue for cities trying to improve their biking environment is
for them to understand the strategies that will be most effective for
achieving their goal of a safer, more sustainable transportation
system. Their cause would be aided by having a better understanding of
the underlying factors contributing to the safety of existing bike
friendly cities. For example, it would be useful to know the key
differences in transportation infrastructure that set bike friendly
communities apart from other cities. Also, is there evidence to
suggest that these differences contribute to safety?
Recently, in the journal Environmental Practice, we published Evidence
on Why Bike-Friendly Cities are Safer for All Road Users, which
examined eleven years of traffic safety records for 24 medium-sized
California cities with various levels of bike use. One goal of our
study was to assess how differences in street and street network
design might contribute to higher numbers of bike users and,
concurrently, to a better traffic safety record.
Initially, we found that the 24 cities in our study could be divided
into 4 distinct groups: 4 high biking cities, 4 medium biking use
cities, and two groups of cities with low bike use (4 low biking
cities with a low traffic fatality rate and 12 low biking cities with
a high traffic fatality rate).
To best highlight the salient differences between the four groups of
cities, we limit this discussion to two of the four groups: the high
biking cities and the low biking/high fatality cities. We found that
the high biking cities averaged 2.5 fatalities per year per 100,000
residents compared to almost 9 deaths per 100,000 for the low biking
cities. Moreover, all classes of road users were at greater risk in
the low biking cities. For people in vehicles, the fatality level was
on average 4 times higher in the low biking cities. This is perhaps
not surprising, since there were more people driving in the low biking
cities.
What is surprising, and disconcerting, is that there were twice as
many bike fatalities in these low biking cities compared to the cities
with many, many more bike riders. In other words, for anyone brave
enough to use a bike in these low biking cities, the risk of injury or
death was astronomical. Conversely the traffic fatality risk from
biking riding in bike friendly cities is much lower than is generally
recognized.
It important to note that this disparity in fatality rates between
cities was not necessarily due to fewer accidents - in fact, the
high biking cities had more fender benders than did the low biking
cities. Instead, the difference in fatality rates between the cities
related to difference in the severity of the crashes that did occur.
In other words, most crashes in the high biking cities resulted in
little or no injury, while a much higher percentage of accidents
resulted in a catastrophic outcome - either severe injury or death -
in the low biking cities with high fatality rates.
Photo: Sign for bike lane.
This is a key finding, because it points to one important difference
between the groups of cities. The results strongly suggest that
crashes in the safer cities are occurring at lower speeds and, as
such, the result of any given crash is less catastrophic. We have not
yet conducted speeds measures in all 24 cities, but in the 6 cities
that we have sampled, the measured speeds on major streets in the
safer cities (the high and medium bike use cities) are significantly
lower than in the low biking cities.
Although the 'safety in numbers' effect might account for some amount
of disparity in vehicle speed, it is likely that a bigger factor is
related to the design of the street and the street network in the
various cities. Our data provides strong evidence for this conclusion.
For example, we discovered that the street network density in the high
biking cities is almost twice that in the low biking cities with high
fatalities.
The cross-section of the major streets was also different. In general,
the streets in the high biking cities were narrower by about 3 ft on
average. This is not a huge difference, but these cities with narrower
streets also do much more within their street cross-sections, since
they have far more miles of on-street parking and bike lanes.
Taken together, these street and street network characteristics of our
high bike use cities add up to an environment that is likely much more
attractive for the causal bike rider.
We also know that the cities like Cambridge, Portland, and New York -
that have had recent success in increasing bike use - have often
taken steps to reduce motor vehicle speeds and volumes on streets with
bike facilities or bike facility crossings. They have done this by
reducing space for cars and adding space for bikes. They have also
employed traffic-calming strategies and focused on providing safe
opportunities for people on bikes to cross the busier roads. In other
words, they have made changes that make their streets more like the
streets in the high biking cities in our study. So it is perhaps not
surprising that these cities have also seen a reduction in the traffic
fatality rate to a level comparable to that in our high biking
cities.
There is also evidence to suggest that just putting down paint to
create bike lanes next to fast moving traffic may not get the job
done. In fact, a few of our low biking cities did have extensive bike
lanes on major arterials. However, these cities have not been
successful and never saw the biking numbers, or the safety
benefits.
Ultimately 'safety in numbers' does not just happen. Instead, our
research suggests that the same strategies that attract bike riders
are the same ones that improve road safety for all road users. Cities
should indeed strive for 'safety in numbers' but before they can get
to that point, they need to create bicycle friendly streets that will
make it comfortable enough for the average Jane and Joe to take up
biking. It is this act of creating comfortable and complete biking
networks that ultimately results in both making cities biking friendly
and, at the same time, making biking friendly cities safer for all
users.
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