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).

Kevin Krizek

About Kevin Krizek

Kevin J. Krizek is Transport Professor, Programs of Environmental Design and Environmental Studies at the University of Colorado Boulder. He was a 2013 fellow of the Leopold Leadership Program and received a 2013 U.S.-Italy Fulbright Scholarship to the University of Bologna (Italy). He is currently a visiting professor of "Cycling in Changing Urban Regions" in the School of Management Science at Radboud University in the Netherlands. Kevin blogs at Vehicle for a Small Planet and can be found @KevinJKrizek . Prior to moving to Colorado, he lived in Minneapolis and St. Paul and was an Associate Professor at the Humphrey Institute of Public Affairs at the University of Minnesota.