Would you spur swimmers to the beaches of Amity Island?

jawsJaws, the blockbuster thriller film from the mid 1970’s was the highest grossing film ever until Star Wars was released two years later. The mechanical shark, the beach scenes on Amity Island, and the music score brought it all together.  The dynamic between the obdurate mayor (Richard Vaughn) and the police chief (Martin Brody) largely revolved around a tension about how to address an activity that, in the public’s eye, has safety risks. Thirty years later, it’s a tension we wrestle with in bicycle planning.

After a supposed shark attack, Brody urged the city to close the beaches; Vaughn ruled otherwise for fear of spoiling the upcoming summer tourist season. Brody was the ultimate hero in the movie, but many bicycle advocates implicitly—or in the case of the Mayor of London[1], explicitly—commend Vaughn’s instincts.

The matter between Brody and Vaughn was informed, in part by a marine biologist. Let’s imagine the biologist confirmed a correlation between the number of people swimming at different beaches of Amity Island and the number of shark attacks[2]. He finds the primary beach is relatively safer to swim because the overall rate of shark attacks is lower (as opposed to secondary beaches). He further postulates that swimmer density causes sharks to avoid those beaches[3]; the number of swimmers modifies the sharks’ behavior. The mayor’s accepts the theory and therefore spurs tourists to the beach[4].

The Mayor of London (along side others aiming to bolster safer cycling) pulls a page from Vaughn’s playbook. His arguments lean on a concept largely popularized in a 2003 study. Peter Jacobsen[5] examined injury rates, pedestrian and/or bicycle volume, and population over time in several different settings. His main conclusion: rates of bicycle fatalities are less when more people bike; on a bicyclist per capita basis, there are fewer crashes with cars in communities where there are more people riding bikes relative to those communities with fewer people riding bikes. Since 2003, many have replicated the safety in numbers analysis with different data, including Minneapolis. If you flip through, “Understanding Bicyclist-Motorist Crashes in Minneapolis, Minnesota,” (released by Public Works Department earlier this year), you find it referenced on pages 3, 7, and 25. The report is a comprehensive account of the bicycle crash scene in Minneapolis and an impressive document. The narrative itself “dances” around the concept and its significance[6].

I know many who side with the Mayor of London but few who side with the mayor of Amity Island. Is something different?

Some analytical weaknesses of Jacobson’s study have been uncovered[7]. Failure to account for confounding explanations has also been discussed[8]. There is the issue of relative risk versus absolute risk. Causality is always called into question when relying on cross-sectional analysis. But the primary rub lies in the build up to the assertion, “the most plausible explanation…is [the] behavior modification by motorists when they expect or experience people walking and bicycling” (Jacobson’s words, not mine, page 208). Text from the Minneapolis report is a bit more conservative, “It may be a sign that motorists are coming to expect bicyclists on certain streets” (italics added).

Are these behaviors similar to how swimmers modify shark behavior? Both theories may be true, but is hard to say with certainty.

Safety in numbers is a banner that is deceivingly easy to march behind. Leaning on it too much to explain something as complex as cycling safety is unsettling. Especially for politicians, it provides a rally cry requiring little investment and gives them an easy out (i.e., let’s get people on bikes and then safety might take care of itself). It shifts the responsibility to cyclists; it too easily takes attention away from infrastructure improvements. The value of the concept, thus far, should lie not in its ability to explain cycling safety but rather as retroactive analysis to show that more cycling fails to increase cycling crash rates.

Many factors influence safe cycling[9]. The real issue is tackling the most important thing St. Paul or Minneapolis (or Anyplace, Minnesota) can do to get more people on bikes in the first place. Part 2 of this post will turn to the role of facilities in luring people to this aim.


[1] The Mayor of London, Boris Johnson’s quote Time Magazine first drew attention to the Jaws analogy (http://content.time.com/time/magazine/article/0,9171,2117236,00.html). But the real acknowledgement goes to the blog post (http://aseasyasridingabike.wordpress.com/2012/10/03/swimming-with-sharks-the-truth-about-safety-in-numbers/) for expanding the JAWS concept and stimulating some of the thoughts here.

[2] I am taking a bit of writer’s license in using this example. The marine biologist’s research in the movie actually sided with Brody’s position; the biologist’s expertise documented the first victim was killed by a shark and that a tiger shark’s autopsy (who was suspected of a second killing) did not contain a human.

[3] The real reason might be that sharks prefer colder water and, owing to currents, the smaller beaches have colder water. Or, sharks like deeper holes and that the smaller beaches are closer to deeper inlets.

[4] In the mayor’s words, “Tomorrow is the fourth of July! And we will be open for business…Now if you fellas are concerned about the beaches, you do whatever you have to, to make them safe. But those beaches will be open for this weekend!” Vaughn fails to give Brody the resources to “do what ever you have to” and sees the solution as getting more swimmers in the water.

[5] Jacobsen, P. 2003. Safety in numbers: more walkers and bicyclists, safer walking and bicycling. Injury Prevention, 9:205-209.

[6] For example, it explains, “the number of bicyclists themselves appear to be improving safety” (italics added).

[7] Rajiv Bhatia Megan Wier (2012).“Safety in Numbers” re-examined: Can we make valid or practical inferences from available evidence? Accident Analysis & Prevention. Volume 43, Issue 1, January 2011, Pages 235–240.

[8] Wesley Earl Marshall, Norman W. Garrick (2011). Does street network design affect traffic safety? Accident Analysis & Prevention, Volume 43, Issue 3, May. Pages 769-781.

[9] Factors that are top on the list include the overall physical environment; the volume and speed of cars in adjacent traffic lanes; the provision and quality of cycling facilities; and, social norms and user awareness (of both cyclists and motorists).

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5 Responses to Would you spur swimmers to the beaches of Amity Island?

  1. Jeff Klein December 13, 2013 at 9:23 am #

    Anyone who has been biking around Minneapolis for a few years will tell you how much better things have gotten as the numbers of riders have increased and cars have gotten more used to the presence of bikers. It’s worth asking what shunting bikers off the streets and on to cycle paths will accomplish. I could believe that they’ll cause an overall increase in the number of riders and will help the situation, or I could imagine that fewer riders on the streets will undo what progress has been made.

    • Bill Lindeke
      Bill Lindeke December 13, 2013 at 1:53 pm #

      picking an old fight here, but nobody is going to shunt anybody. there’ll be more people on the streets, more in the bike lanes, and more on the cycletracks. there is a lot of pent-up demand for bicycling in the city.

      • Jeff Klein December 13, 2013 at 5:03 pm #

        I’m sorry if it came off that way but I’m not really trying to “pick an old fight”. I’m really trying to have an open mind about the cycle path thing, particularly given that I really love the content on this site and agree with the various writers on essentially every other issue. And my question was honest, I wasn’t posing it as rhetorical. Given the content of the article, it doesn’t seem like a crazy question to pose.

  2. hokan December 13, 2013 at 11:30 am #

    Another explanation might be Smeed’s Law.

    http://en.wikipedia.org/wiki/Smeed%27s_law

    Somewhat restating what Smeed said, “mode share increases as safety increases”.

    The difference between that and Safety in Numbers is the question “does mode share impact safety” vs. “does safety impact mode share”.

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