Mark Stehr

For this post, I interviewed Professor Mark Stehr. Associate Professor Stehr is the Assistant Director of the School of Economics at the LeBow College of Business at Drexel University.  He is a health economist. His most recent work is at the intersection of health economics and behavioral economics.

In “Intended and Unintended Consequences of Youth Bicycle Helmet Laws,” Professor Stehr and his co-author, Christopher Carpenter, study a policy designed to protect people. In the US, 21 states have passed laws that require at least some people to wear helmets while riding bicycles. In most cases, the laws require those under 16 years of age to wear helmets. The perspective that this paper takes is that the goals of the laws are to save the lives of these children if they are hit by a car, or involved in another type of serious accident, while riding their bike.

This isn’t the first paper to look at this issue. Previous work, by Grant and Rutner (2004), found that helmet laws were in fact effective in saving the lives of bicycle riders. That paper used data from 1990-2000. In this paper, the authors confirm this result using data from 1991-2005. They find that helmet laws reduce fatalities from bicycle riding among youths by almost 19%. 

The authors are able to find this result by comparing the states with the helmet laws to those without the laws. In addition, they provide evidence to bolster the credibility of this result: the helmet laws do not significantly affect fatalities among 16-30 year olds, who are not required to wear helmets.

Those of you who live in states with one of these laws, and/or who have children, might not be surprised to hear the next interesting result in this paper. Not everyone is adhering to the laws. The authors are lucky enough to have access to two different surveys, one of parents and of children. Among parents, 35% say their oldest child never wears a helmet; but 83% of children say they never wear a helmet.  

Yet, the helmet laws are still effective. This is partly because of the most interesting finding of the paper: an unintended consequence of the helmet laws is that they cause some children to stop riding their bikes altogether! Helmet laws lead to a 3-5% reduction in the probability of a youth riding a bike in the past year. 

Why does this happen? It’s likely that, when asked to don a helmet, due to concerns about comfort and looks, some kids decide they would rather not ride a bike at all. Evidence from surveys shows that kids complain about the fit of helmets: they are often tight and sweaty. And kids are always concerned about looking “uncool” while wearing a helmet. If you ask me, this is probably the #1 reason that kids aren’t wearing helmets.

It would be very interesting to know what activities these kids take up, instead of bike riding: do they skateboard (possibly without helmets)? Do they play video games? In some cases, kids use bikes for transportation; and so they might be taking the bus, walking, or otherwise getting a ride. This paper acknowledges these questions, but it’s not a goal of the authors to answer them. (Note to self: future post?)

I asked Professor Stehr whether he thought the state governments -- the policymakers in this case -- might have been able to foresee that this unintended consequence could happen. He told me that he didn’t know for sure, but that this example is very similar to the implementation of motorcycle helmet laws. There is already lots of evidence that these work in reducing fatalities, and that these motorcycle riders don’t like wearing the helmets either. 

Lastly, I asked Professor Stehr what he would recommend policymakers do differently, now that we know about these unintended consequences. He thinks that manufacturers should make bicycle helmets that kids think are cool and stylish. He also has general recommendations for bicycle safety besides helmet laws. For example, he told me about his experience riding his bike in Philadelphia:

“I’ve been riding my bicycle in Philadelphia for 15 years. I used to have people yell and honk at me to get on the sidewalk where I belong. Of course, bicycle riding on the sidewalk is illegal. Bicycles are supposed to be on the roads. In Philadelphia, they have painted what they call “sharrows,” which are a picture of a bicycle and an arrow, on the street, all over the city, to let motorists know that bicyclists are supposed to be there. I think it’s worked. No one has yelled at me in years. It’s made me feel safer.” 

I asked Professor Stehr about some of his other recent work. He told me about a field experiment he is working on with several co-authors. You can read about it in this working paper. A field experiment is like an experiment in the lab, but it takes place out in the real world. In this case, the experimenters want to understand how to best use incentives to encourage people to exercise. For example, they compare paying them more up front to spreading out payment. More broadly, Professor Stehr has been researching how to incentivize people who genuinely want to change their behavior – like eat healthier or quit smoking --  but repeatedly come up short.

Let’s talk! I would love to know what you think about this example of unintended consequences. Please submit comments and questions.

Lucas Davis

For this post, I interviewed Professor Lucas Davis about his paper, “The Effect of Driving Restrictions on Air Quality in Mexico City.” 

Professor Davis is an Associate Professor at the Haas School of Business at the University of California, Berkeley. He is also the Jeffrey A. Jacobs Distinguished Professor in Business and Technology there, as well as the Faculty Director of the Energy Institute at the Haas School of Business, and a Research Associate at the National Bureau of Economic Research. He specializes in the economics of energy and the environment. 

In 1989, the government of Mexico City enacted a policy to control air pollution. The policy, called Hoy No Circula (HNC for short) is still in place today. The government was very concerned about the extremely high levels of air pollution, and they thought that placing a restriction on driving would be a good way to reduce it. The policy bans most drivers from driving their vehicle one weekday per week. For each vehicle, the last digit of the license plate determines which day the vehicle cannot be driven.

In Professor Davis’ own words, “I lived in Mexico City in the 90s, and I coped with this program on a routine basis. I found it to be a huge hassle. . . I always assumed that if there was so much pain being inflicted by this policy, then there must be must be some evidence of its effectiveness.” He carries out a very thorough search for such evidence, but he just cannot find any.

First, he looks at the daily pattern of pollution after the policy is enacted. Carbon monoxide, for example, which vehicles emit, follows a typical rush-hour pattern. It peaks at 9 am and at 6 pm. He wants to be thorough, so he also examines several other pollutants, including nitrogen dioxide, ozone and sulfur dioxide. None decreases after the policy. He also finds that drivers seem to have simply switched to using their cars more on weekends. 

When I asked Professor Davis what the unintended consequences of this policy were he said, “The problem with telling people that they can’t drive their car once a week is that it means they are going to look for substitutes on that day; and anecdotally, long before I wrote this paper, I was hearing that people went out and bought more cars.”  He goes on to say, “Now if, for example, I have a third car, I can loan it to my teenager the other days of the week, and increase my family’s total driving.” 

Professor Davis looks for solid evidence of these anecdotes he was hearing. He finds it. First, he finds that gasoline sales did not decrease after the policy. He also finds evidence that both the number of registered vehicles and sales of vehicles increased after HNC. And, think about it, what kinds of cars do you think people would go out and buy? He discovers that people bought mostly used vehicles, not new ones. So, most of the extra vehicles on the road were high emissions vehicles. Of course, this ended up contributing to air pollution and gasoline consumption! 

How else did people react to this policy? He has some evidence that they took taxis more; and the taxis in Mexico city at the time the policy was implemented consisted of mostly relatively old, high-emissions vehicles, also making air pollution worse instead of better. 

But wouldn’t people take public transportation? It was a stated goal of the program. It’s a very good system (subways and buses), but he finds no evidence that people switched to it. In fact, Professor Davis’ evidence is that ridership decreased slightly. He thinks that this might be because the people who drive to work are from the middle and upper classes and hence value time highly. They therefore took taxis and bought additional cars instead, as we just discussed.

Professor Davis struggles to identify any real winners from this policy. He thinks that everyone, except maybe the taxi drivers, has suffered in Mexico City, because of this policy. It’s been a huge inconvenience, and it just hasn’t worked.  

But, it turns out, several countries have this kinds of policy in their big cities, and more are planning to implement it, including Delhi and Paris. Several large cities in South America also still use it. So I asked Professor Davis what types of policies he thinks should replace this policy. One suggestion he had was better emissions testing. He said that emissions testing is sometimes too lax, and sometimes even fraudulent. With tighter standards, we can keep offending vehicles off of the roads. 

Let’s talk! I would love to know what you think about this example of unintended consequences. Please submit comments and questions.