Portland Safety Data

Last week Grist had a great article about “safety in numbers” for cyclists. Research within the US and around the world has shown that as bicycling rates go up, the total number of crashes stays flat, resulting in a significant decrease in bicycle crashes per trip. While this might initially sound counterintuitive, the premise is pretty simple. The more cyclists a city has, the more drivers expect cyclists, and drive accordingly. Similarly, as bicycling rates increase, drivers are more likely to be cyclists themselves, causing further behavioral changes that improve the safety of bicycles.

The phenomenon, dubbed “safety in numbers,” was first identified in 2003, in an academic paper by public health researcher Peter Jacobsen [PDF]. After being asked by officials in Pasadena, Calif., if their city “was a dangerous place to bicycle,” Jacobsen began looking at crash data from various communities where bicycle ridership had fluctuated over time.

What he found surprised him: The number of crashes involving bikes correlated with the number of riders in a community. As ridership fluctuated, so did the crash rate. More riders, fewer crashes; fewer riders, more crashes.

This happened too abruptly, Jacobsen decided, to be caused by slow-moving factors like infrastructure development and cultural change. Bicycling becomes safer when the number of riders increases, he concluded, at least in part because the number of riders increases.

The inverse happens, as well. One data set Jacobsen looked at covered 49 years of biking history in the United Kingdom. Those numbers showed that cycling became safer during the oil crisis of the 1970s, caused by the OPEC oil embargo. Once the crisis ended, both ridership and safety dropped.

More after the jump.

For me this brings up two points about bicyclist safety from a public health perspective that almost always get glossed over. First, helmets are a distraction when it comes to the overall safety of bicycling in a city. Helmets are solely about harm reduction. They do not prevent collisions from occurring, they simply reduce the severity of injury caused by blunt force impact to the head. Put another way, even if 100% of cyclist wore helmets it would do nothing to reduce the incidence of crashes.

Second, the primary purpose of bike facilities, in my view, is about getting people that are “interested but concerned” out biking. This segment of the population has the largest immediate growth potential and are what makes the difference between the left and right in the graphic above. Bike facilities entice these types of people to bicycle, creating a virtuous cycle of improved safety, real and perceived, because of “safety in numbers”. This leads to more people biking, and so on and so forth.

This is what you find in every Western country with high bicyclist safety. Lots and lots of people riding, without a helmet in sight, on high quality segregated bike facilities.

70 Replies to “On Bicyclist Safety”

  1. Hmmmm. If you add a couple of side-car bikes, a trailer with sidecars, and ‘blow-up’ riders all around, then there’s 6 of you. It could be the Mayor McGinn rolling staff meeting, where he wins all discussions – AND in complete safety!

  2. The interesting thing from this column is the last paragraph quoted from the article about bicycling gaining in popularity in the early 70’s then falling off. I remember bicycling in Seattle being very popular in the 70’s, also, when I rode a bicycle myself. But, as in the United Kingdom, I suspect bicycling in Seattle really fell off in the late 70’s. Is there any data about trip share by bicycle in Seattle in the 70’s?

    I consider the current push by the city of Seattle to get people to ride bicycles to be a “fad”, like so many other things. It will likely last for a few years, then people will get tired of it. The next “fad” will probably be something like electric cars, motorcycles and scooters, which will be very “clean” and a lot more practical than riding a bike.

    Just like people got tired of riding bikes in the 70’s, many of the new bike riders in Seattle are going to get tired of bicycling pretty quickly and go back to motor vehicles, which are a whole lot better way of getting around than riding a bike.

    1. “fad?” I’ve been commuting for over 40 years if you count my riding to school. I’ll let you know when the novelty wears off. Until then days like this week are fantastic bicycling commuting days.

      Secondly I see automobiles starting to have to pay more for the streets they use. This city has a huge backlog of work to repair the streets they have and I see additional taxes on automobiles paying for that via parking taxes, gasoline taxes and additional property taxes. When those taxes come up for a vote, to get them passed the bicycle lobby will be pushing for additional bike boxes and bike lanes. That’s going to add to the ease of bicycling.

      1. How stupid, if the city has a “huge backlog of work to repair the streets,” to waste any money on bike boxes and bike lanes. I believe a large majority of Seattle voters agree with me on this.

        There is always a tiny percentage of people who use bikes long-term for transportation. Then there is a larger group who tries bicycling for a while, then gives it up for various reasons.

        Biking is in a temporary uptick in popularity now, as it was in the 70’s. [off-topic]

      2. No Norman a majority do not agree with you because they passed Bridging the Gap. Bike facilities are *cheap* and are often done in conjunction with repaving or re striping projects will be done anyways. Portland built it’s *entire* bicycle system for ~$60 million.

      3. Driving to work in Seattle is down 7.7% from 2000-2009 while cycling is up 59.0% and transit is up 10.9%, maybe it’s driving that’s the fad whose novelty is wearing off.

      4. Given a “huge backlog of work to repair the streets” spending money to encourage bicycling makes sense. Bikes will cause a lot less additional wear and tear on the streets, which would make them last longer than if those trips were in cars.

        Improving bicycle infrastructure ought to reduce the “various reasons” that cause new people to stop cycling.

      5. I’m not sure about the fad or lack thereof – plenty of people still see bicycling as corny or dumb or slow as a commute method, and there are plenty that are trying it who have never done it before.

        Thing is that a lot of commutes in Puget Sound are taking almost as long by car as they do by bike. Particularly heading west to east. The number of cars on the road continues to rise. I spend 2 hours or more a day in my car, or 2.5 on my bike. it’s driven me to abandoning car commute 4 days out of the week, either to bus, or to bike (via the SRT). 18.5 miles each way is no small potatoes and it would be a nonstarter if my job didn’t have showers.

        While I welcome the additional infrastructure so that I can better manage errands on the weekend, continuing the trend is going to require a refocus on incentivizing employers (or building owners) to provide additional resources for cycling commuters. Without that (or massive gas hikes), I do think we’ll see a retreat from the current peak usage.

  3. Yeah, I took up the “fad” 10 years ago. Seems like I’m not really getting tired of it. In a prior post, Norman, didn’t you purport to be a cyclist? What gives? More people are riding because it’s truly easy to get around. It takes me half the time to get to/from work via bike as it would via bus. Yes, it’s faster “some” of the time to use a motor vehicle, but not all the time. Travel times via motor vehicle are only increasing…due to all the other motor vehicles trying to get somewhere at the same time. While I don’t ride a bicycle all the time to run errands, I do try to choose it or my own two feet rather than drive. It’s not about “fads” it’s about people making choices that are different from the mainstream.

    What was the population of Seattle in the 1980s when cycling (per Norm’s deduction) dropped off? Was cycling really truly “popular” here in the 1970s? I’m willing to bet that IF cycing dropped off after some abberant “fad” and cars became more popular it was because driving was cheap and easy – which is not the case today.

    1. 1973 is still the record year for the number of bicycles sold in the U.S.

      I never purported to be a bicyclist. I rode a bike when I was a kid and when I was in college. I gave it up when I graduated from college.

      What makes you think driving today is not cheap and easy? It certainly is for me.

      1. Driving is cheap and easy? Adding insurance, cost of your vehicle maintenance, licensing, gas costs, parking costs, general taxation costs (most road taxes are taken from the General Fund which includes property taxes, sales taxes, etc., so we all do really pay for the roads)? I know that my household can only afford one car. It is expensive if you add up all the residual costs. Of course, if your income is high, the costs won’t necessarily effect you as much as someone else.

      2. Found this cool web calculator to show you what the cost per driving is.


        Looks like my 28 mile commute if I drove would be around $9,800 dollars.

        Now my bicycle is not free, but if I include all that fancy wool & spandex and extra cookies ( I treat myself to one a day that I ride), new tires & tubes I’m only at $1,800 year.

        Cheap, is in the eye of the beholder but bicycling looks lots less money than driving.

    1. Well there is the disagreement over the allocation of funds. But as a bicycle commuter, I’d prefer to invest in elevated transit so that the two modes don’t collide.

      1. Sometimes I wonder if ETC (monorail) was the right way to go.

        An elevated monorail to take people across the the whole region with a limited number of “express stops” ( Seattle, Renton, Bellevue, Kent, Everett) and then bus feeders for the rest.

        As it stands now we bought the nation’s most expensive bus. A slow, inefficient and dangerous bus…

      2. The Link isn’t dangerous. What is dangerous are oblivious drivers who think they can beat the train and break traffic laws.

      3. Well the technical Monorail killer for the region was the height of the cars. There are only one or two models that would have fit through the I-90 tunnel. And we needed a high capacity train as it was designed.

        The second problem is the fire access walkway requirement meant that the elevated track was visually large.

        However for the in city routes, West Seattle & Ballard, the technology was reasonable.

        Where IMO the monorail should have been built was in place of the South Lake Union Trolley. That’s a slow dangerous bus on rails.

  4. Dear cyclists who read STB,

    Please stop running stop signs. I don’t want to kill you, but I will if I hit you with my truck.


    1. Thanks for watching out for us Luke. I’ll be sure to watch for you. Believe me, neither of us wants to meet in the intersection.

    2. Will do. As you are concerned about the safety of cyclists maybe you can start trying to convince your fellow drivers to stop speeding, and start yielding to pedestrians and cyclists in crosswalks, using turn signals and giving us plenty of distance when you pass.

    3. I don’t run stop signs. I will continue to holler loudly (and ineffectually) at my fellow cyclists who do, but how about you stop imputing the poor behavior of some cyclists to the rest of us.

    4. I completely agree. Bicyclist have the same rights and responsibilities as motorist and more of one requires more of the other.

      1. Adam,
        I disagree with you. Car and truck drivers have a greater responsibility to be safe, because they are much more likely to cause injury or death with unsafe behavior. Take the analogy of a hunter. A hunter carrying a loaded gun has a much greater responsibility to use his weapon safely than a hunter in the same party carrying a sling shot.

      2. Brian i completely agree. In my hast I didn’t fully flesh out what I wanted to write. Your point is excellent and I have been meaning to write about it in the context of a vulnerable user law. What I really wanted to say was if drivers want cyclist to obay laws more that also means cyclist need their own facilities and drivers need to respect their rights to use the road. Sorry I didn’t fully put that out.

      3. I’m definitely with you there.

        Have you guys discussed the implications of the helmet law with regard to your model bike share program?

      4. Are you talking about the UW bike share study? They actually had a good section on it in the report, but I think the conclusion wasn’t too realistic. Bike share systems just aren’t compatible with helmet laws.

    5. Dear drivers who read STB,

      Please stop driving over the speed limit. It’s against the law and dangerous.

      By the way, I commute ~25 miles daily on my bike, and I can’t remember the last time I’ve run a stop sign or stop light. I wish all other cyclists would do the same! It would be much safer for all of us.

    6. For every cyclist you see running through a red-light, there are many more who actually obey the traffic laws. Think of it this way – you as a driver are driving down the highway. You don’t come rushing home to your significant other or friends saying “Oh man, you won’t believe what I just saw! A driver properly used their turn signal to notify other drivers around him on I-5 that he/she would like to merge into the lane to their right. After a few seconds, they then make the lane change flawlessly!”

      People, cyclists and drivers alike, only take note of those who break the rules and then apply that to everyone else in that demographic. Honestly, take the time to look and notice those who do obey traffic laws. I think you’d be surprised how many cyclists are law abiding citizens. And in the end, we are just another human trying to get from point A to point B; it doesn’t matter if we’re behind a the wheel or on top of two.

    7. I don’t run stop signs, so it would be hard for me to stop running them. But thanks for the concern.

    8. Can STB look at its access logs and forward Luke’s IP address to the Seattle Police so they can do an investigation?

      At this point he’s announced in public that he wishes to do intentional harm to bicyclists on public roadways.

      1. Calm down, Luke does not say that he will intentionally hit bicyclists, just that if they run stop signs it’s possible that the outcome of the collision will be a dead bicyclist.

        Since we can infer that Luke has been driving for some time, and that bicyclists have been running stop signs for some time, and we know that there have only been two cycling deaths in the last year, neither of which Luke was involved in, that Luke had he wanted to hit a bicyclist, would already have done so, and has not.

        That what he typed was a typical blog posting of terseness, intended to ask cyclists to be careful.

    9. Dear oblivious drivers who run red lights,

      Please stop running red lights because:

      1) I don’t want my wife to be a widow if you hit me while I’m riding my bicycle

      2) I don’t want to fill out the paperwork after killing you when I’m driving my bus. Enduring the trauma of killing another human being wouldn’t be a picnic either.

      Thank you,


      P.S. Seriously, Luke, I see a LOT more stupid and dangerous things done by drivers of automobiles than cyclists. You should be griping about all errant drivers/riders/operators – not just picking on cyclists, who, for the most part, are risking their own lives, not those of others.

      1. Hmmm… That’s weird, I didn’t think I posted this version. I had edited the PS out but there you have it. The powers that be can delete this version or let them both stand :)

    10. Dear oblivious drivers who run red lights,

      Please stop running red lights because:

      1) I don’t want my wife to be a widow if you hit me while I’m riding my bicycle

      2) I don’t want to fill out the paperwork after killing you when I’m driving my bus. Enduring the trauma of killing another human being, no matter how stupid, wouldn’t be a picnic either.

      Thank you,


  5. Another way to interpret the data is that the vehicular lion’s population is stable, and only needs to cull a certain of bicycle gazelles from the herd every year. So as the population of gazelles explodes, the odds of any particular gazelle of being eaten declines.


    I have a hypothesis that helmet laws are actually unhealthy, and am pondering a way to test my hypothesis, but I don’t think the data is readily available. I suppose I could do an ecologic study, doing inter-country comparisons, but the limitations are great.

    My thoughts as to why this would be:

    Drivers show more disregard and find it easier to dehumanize helmet riding bicyclists.

    Teens, who are more apt to ride when older if they begin riding now, look at helmets as uncool.

    The benefits of riding over a lifetime vastly out-weigh the benefits of wearing a helmet, if it were an either/or proposition, in terms of mental and cardiovascular health.

    Bike sharing programs don’t work in areas with helmet laws.

    If anyone can think of some retrospective datasets that could help answer this question, I might pursue it.

    1. There was a British study that showed that drivers gave less room to cyclists wearing helmets than they did to those without helmets.


      I think the main way that bike helmet campaigns hurt cyclists is in their efforts to keep cyclists safe they everybody that cycling is dangerous. This causes fewer people ride which in turn makes cycling more dangerous.

      1. When the researchers donned female wigs they got more clearance, 14cm (5.5 inches) more than apparent males in helmets. They did not report on what a skirt and helmet combination would do. The author was hit by a bus and a truck during the experiment, and was wearing a helmet both times.

        That’s dedication! I wonder why they didn’t use actual women instead of a wig?

    2. I have also seen a study (can’t remember where) that drivers give women and people with kids with them more passing room.

    3. I have actually read that helmet laws have been shown to discourage cycling slightly, which means, if the power of safety in numbers is strong enough, helmet laws may actually be counterproductive to reducing incidents.

    4. I think it is a wrong choice to throw out helmet laws because people think wearing helmets is uncool and don’t like doing it. It’s the same reason we have speeding and seat belt laws, people are often bad at making risk assessment. Therefore, when it doesn’t unduly hamper the individual freedom, there are laws to protect people from themselves.

      It is a huge expense to care for someone who had a traumatic brain injury which could have been prevented if they wore a helmet. Really, the only reason helmet use is that people don’t like wearing them. It’s like if breaks were uncool and people didn’t like having them on their bike, we would have to make a law that a bike must have some sort of break. If anything, helmet laws help push helmet use into the norm, which saves lives.

      If there was an inflatable suit that looked like a clown but provided significant protection against being run over, I would also wear that. Your own safety should be more important than looking cool.

      1. Well, speeding harms others, whereas seat belts and helmets protect you from yourself. I wear I seat belt and a helmet (mostly), but I don’t agree that there should be a law mandating that I have to. If I want to live dangerously, I don’t think we should stop me from doing so just because our aggregate insurance rates might blip up. Taking personal freedoms shouldn’t be that easy to justify.

        And I’m in public health, study statistics, and understand more than most people the true risks.

        I just think we should educate instead of mandate.

      2. Well the helmet law is a “public health” law, and last time I looked the fine was on the order of $25. Which is roughly the cost of an inexpensive helmet.

        The trouble with relying on people to act in their best interests is that when they don’t, the aggregate cost is not just higher insurance premiums, because up until recently you weren’t required to have health insurance. So the cost of care was transferred to the tax payers.

      3. Yeah, I get that. I understand it makes sense, both economically and in terms of injury prevention.

        I just disagree we should take away people’s right to choose how risky they want to be with their own bods.

        Our rights be balanced so as to minimize the infringement of others rights and well-being. We shouldn’t be taking away citizens’ rights using abstract arguments on how much having that right might mean in terms of a dollars. If we start down that path, you could argue any of our rights and civil liberties away, because they all would have some economic impact and somebody else.

      4. Greg,
        The problem is that helmet laws are so counterproductive in other ways that they probably end up causing more net harm than good. This sort of thing is difficult to model. But here’s the schematic way of looking at it:

        On the positive side, a helmet (if worn properly) will help prevent traumatic brain injury in certain kinds of collisions. Such a collision would have to be low speed and not involve too many rotational forces on the head; helmets do not prevent axonal injury in situations where the brain is spun around inside the skull.

        On the negative side, we have good evidence that helmet laws substantially lower the overall rate of cycling in a community. When this happens, cyclists often turn to less-healthy ways of getting around, like driving. Unfortunately there are many many negative impacts to increased sedentary activity which have been discussed here at length. Among these are increased rates of coronary disease and obesity.

        In addition, drivers are probably more aggressive around helmeted riders, increasing the risk of collisions. Also, cyclists wearing helmets may be more aggressive around cars, although I don’t know if there are data to support that.

        Finally, on a more theoretical level, bicycling may improve general psychological welfare. For example, exercise is strongly protective against depression. Bicyclists anecdotally do not seem to suffer from road rage to the same extent that car drivers do.

        In the end, if we were trying to decide if helmet laws were a good idea, we would need to balance the reduced rate of traumatic brain injuries in a small number of collisions against multiple other kinds of negative impacts to a very large population. Even if we had perfect data and a perfect model, we would still need to decide which is more important: preventing a traumatic brain injury in a single person, or causing many people to die unhappier at a younger age. They’re both bad: picking between them is a value judgment.

        We don’t have anything close to a perfect model but there is certainly good evidence to suggest that helmet laws are probably net harmful. Given this, I think the state should stay out of people’s business. If you want to wear a helmet, no one is going to stop you. For my commute on the Burke, I’d prefer to leave mine at home.

      5. “helmets cause fewer people to ride.”

        I’ve seen those studies, but they don’t fit with my personal observations.

        The reason my coworkers use to not ride, is:
        “too far”: ie. They live over 20 miles from work.

        “Weather”: But that’s because they view the weather from inside their car where it’s warm but outside the wind is blowing 60 and raining. (because they are driving 60mph through a rain storm!) And often they don’t own the better rain/wool gear that makes riding if not pleasant at least tolerable.

        No one has mentioned the hassle of a helmet.

        On the wear the helmet crowd side:
        My SO works at Harborview up where they do reconstructive surgery on the face. From her perspective, it’s wear a helmet! Is there really any question about it?

        We legislate all kinds of safety stuff because in practice the loss of the liberty to choose to be stupid was a bad bet for society.

        If you would be willing to a) pay out of pocket for any major injury which could have been precluded by your wearing a helmet, or b) be euthanized and removed from the gene pool, then maybe I’d buy the argument that your personal freedom to choose should allow you to be stupid. But since as a society we are unwilling to either a or b, instead we’ll patch you up as best we can and share the cost among all tax payers then you have to wear a helmet. No matter how unhappy it makes you…. eat a cookie, you’ll feel better.

      6. Gary,
        By your reasoning, all car drivers should wear helmets all the time, because wearing a helmet is likely to help prevent traumatic brain injury in the event of a collision.

        Also by this reasoning, all pedestrians should wear Kevlar suits if they ever venture off the curb and into the crosswalk, because a Kevlar suit is likely to help prevent injury in such a scenario.

        Obviously these are extreme positions, yet they are logically consistent with your argument.

        I would reiterate that because the helmet law very likely decreases the general welfare, and because it definitely represents an imposition of the state on individual freedom, we should abandon the law.

    5. When reading the article, and getting to the mentions of helmets, I had to go back and re-read to try to understand what helmets had to do with the point at hand. The answer is nothing. No-one is saying helmets reduce the number of crashes. They reduce the consequences. I always wear a helmet riding. I am fairly indifferent to helmet laws, but one place I think they have benefit is getting young people over the hurdle. It gives them the out of saying they are wearing it because they don’t want to have to pay a ticket (or better yet another ticket). It was a good part of the argument for my (young adult) son, and has had an impact on some of his friends. He got a more “cool” looking helmet (which I happily paid for) but now after borrowing mine he is thinking he may want to move to a more ACTUALLY cool, and light, one.

      1. The point about helmets is that simply saying “I’m safe, I wear a helmet” is about as far as our discussion about safety goes. That is just the tip of the iceberg when it comes to safety and other issues like knowing how to safely bike or having lights at night is very important if not more important.

      2. OK, there are lots of laws on the books which I and virtually everybody ignores regarding lights and reflectors. I’m not sure it’s even legal to run a flashing red tail on a bike but I do it anyway it if think it will help. I always wear a helmet (OK almost always, big climbs in excruciating hot weather I’ve been known to strap it to the handlebars). I’m a big fan of wearing a helmet. I’m not so big of a fan of legislating against stupidity. I’ve seen a big increase over the last year in bike commuters. I’ve also seen that the majority of non-helmet wearers don’t know what side of the street they are supposed to be riding on. Let’s enforce that law before we crack down on helmet laws.

      3. @ Joeski I wanted to add that in most countries with high bike safety helmet use is required for kids under a certain age.

        @ Bernie From my understanding most or all European counties require that cyclist have front (white) and back (red) lights on their bike. In Denmark these lights (http://www.youtube.com/watch?v=Bzef_xPWuTg) are everywhere. I would say 50% of bikes have these. While I was in Copenhagen I actually pick one up myself because I didn’t think I would be able to get them here, although I just recently saw them at Greggs.

      4. Adam, I’m all for lights, reflective clothing, etc. I run an HID in the winter that I have to be careful with not to blind on coming motorists. I run an LED strobe front and blinking red taillight at dusk. Blinking handlebar ends are another great invention. I don’t worry about a white front reflector and I don’t have the required spoke reflectors but I’ve got lots of reflective clothing including shoe covers. I’m a fan of helmets. Overall I’m a fan of helmet laws but I can see where there might be some room for improving the law. Definitely a requirement for children should stay on the books just like we require car seats and booster seats in automobiles.

  6. Agree with Biliruben: the bicycle crash rate has remained constant over 18 years. This could just mean that 18 years ago motorists were already injuring as many cyclists as they wanted, or at least the numbers were constrained by factors other than number of available cyclists. It must have been a target-rich environment already: adding more cyclists hasn’t significantly changed the ability of motorists to seek them out and run them down.

    1. I was posting tongue-in-cheek, but it could be that the number of negligent drivers on the road, which can presume to not increase much, is more of factor than the number of cyclists.

  7. I have noticed that it is easier and relatively less onerous to ride a bike these days. Much more so in the past 2 years. There’s more infrastructure (please, add more in the south end SDOT!) and drivers seem to be more aware of cyclists in general. Seattle is far from Portland’s numbers, and I’m not sure we’ll ever get that cycling share due to our terrain, but it’s indeed better out there.

    1. Portland’s terrain isn’t necessarily any more condusive to cycling than Seattle’s. For example, the West Hills are virtually an impassable barrier for casual cyclists to west side suburbs. It depends on where your commute takes you.

      1. I don’t think Seattle is equal to Portland in terrain. You point to West Portland for hills. Everyone points to West Portland. Well, I’m pointing to East, SE, West, SW, North, NW, etc. in Seattle where you will ecounter hills. The neighborhoods for the most part are built on top of said hills. Portland’s neighborhoods are located on the flats. Much flatter than Seattle. If you look at https://seattletransitblog.wpcomstaging.com/2010/04/20/portland-and-seattle-bicycle-analysis/ it’s easy to see. You *will* get more cyclists in flatter terrain; it’s easier to bike. Thus, some of the explanation for higher riding rates in the Ballard/Fremont area…my gosh, I have ridden there and to Wallingford and it’s much easier than riding home to my place in West Seattle. So much so I was laughing out loud at the ease of it. Portland is much more like the Ballard area. A simple terrain map shows that.

  8. Actually, helmet use may be the cause of the improvement: if people are in a crash and aren’t injured because they are wearing a helmet, they may be less likely to report the crash. So

  9. I can see how the results you show work. The problem were I live is there is no bike comuters and very few riders so how do you get the drivers to be safer if none of them have the desire to ride bikes? I would love to get an answer to that one!

    1. The trend line is the rate of crashes, crashes/traffic. The number of crashes has remained constant while bicycle traffic has increased, hence the declining crash rate shown by the trend line.

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