Wednesday, March 2, 2011

The Montreal evidence for cycle track-to-street safety

A recent paper on Montreal cycle tracks has drawn a lot of publicity from the official news sources and blogs without an appropriate amount of skepticism for the results. 

Briefly, the authors compare six cycle tracks to reference streets to compare the relative risk of cycling on those streets.  Exposure is determined by a two-hour simultaneous count on 2009 on the streets.  The number of injuries is tabulated from emergency medical reports (EMR) from 1999 though 2008 during the cycling season; but are subsequently adjusted by estimating the number of intersection injuries from bicycle/vehicle collision police reports.  Aggregating over all six cycle tracks they conclude that the relative risk of cycle track streets to the reference streets is approximately 0.72.  Moreover their two-hour simultaneous count estimated 2.5 times more cyclists on cycle track streets.

Fundamentally, the relative risk measure is only valid if the cycle track and reference street have similar qualities and would face similar risks if not for the cycle track.  Below are person-views -- roughly at the beginning of the focus area -- with maps of the first comparison in Table 1 of the paper.

St. Denis ... the reference street

Brebeuf ... the cycle track street

Personally, I think a picture is worth a thousand words.  In the interest of completing the point, a cyclist traveling northbound on St. Denis has to deal with several more lanes of motor vehicle traffic including opposing direction traffic -- left turns into driveways, u-turns, buses and so on -- in a commercial area.  Moreover, it is unclear to me what type of injuries are in the EMR data; but if pedestrians collisions are included then the St. Denis cyclist would have people shopping and dining in the area.  Wider roads also tend to have faster motor vehicles which increase the likelihood of a serious injury.  As one might expect, Table 3 show that the relative risk to motor vehicle occupants is significantly different from 1.  In short, to find no statistical difference in relative risk would be far more surprising than their estimate.  Identifying any effect from the cycle track is highly unlikely, in my opinion.  Just to be clear, I picked this comparison because it was listed first.  One could grind through the other cycle track-reference street comparisons to be more thorough but I suspect that the point has already been made.  Others have done this precise task.  If and when they write up a paper or webpage, I will be sure to link it.

In large, we can probably stop here with regards to raising a serious level of skepticism about the study.  However, there are also some serious questions about the data underlying the relative risk metric.  The exposure data was collected during a two-hour period on a day with mild weather.  That data was used to compute the exposure ratio for the cycle track and reference street during a 10-year period of cycling seasons.  Now, if we ignore the huge difference in reference periods, we should consider whether the population of cyclists on the cycle track and reference street are similar enough that this remains a good estimate subject to changes in weather, time of day, and other random perturbations over time.  Again a picture is worth a thousand words.  Does anyone expect similar cyclists on the two pictured streets above?

As I mention above, the EMR injury data that occurs at intersections is adjusted for the expected number of cyclists originating from the cross street instead of traveling on the streets of interest.  The authors estimated the fraction from the cross street based on tabulations of police reports which overlap but do not fully cover the same reference period.  But do EMR and police bicycle/motor-vehicle collision reports have the same meaning?  Certainly they are correlated, but simply based on anecdotes in the US, when a police report is filed seems to vary considerably and could differ from say whether an ambulance is sent to the scene.  For instance, until a recently, an Arlington police report would only be filed if the cyclist in a collision with a motor vehicle went to the emergency room despite emergency personnel being sent to the scene.   Note that part of this uncertainty is about my limited understanding of Montreal/Canadian laws and data: It could be the case that the relationship is well understood.  But the short blurb in the paper omitted a thorough discussion and cited a paper about New Zealand.  Consequently, I couldn't tell whether their conclusion was based on their own tabulations/local knowledge or the New Zealand paper.  Nonetheless, besides the potential bias regarding the share of intersection EMRs originating from the cross street, modeling this component also induces a bias in relative risk due to the variance of the estimated shares.  At the very least, it would have been prudent to do some robustness estimates by using police report collision data only to compute the relative risk ratios.  There are 340 and 191 EMR injuries for cycle track and reference streets using the authors' method.  If the overlap is high then one could probably get good estimates to make comparisons.

In the interest of brevity, I want to avoid nit-picking another dozen points on the paper and instead make a few general points relevant to cycling advocacy.  One, many discussions about safety conveniently ignore notions of convenience and speed.  Pro-facilities people gloss over increased conflicts at intersections and slower travel.  Vehicular cyclists (VC) tend to ignore the ability for skilled cyclists to use something like a bike lane to pass motor vehicles stuck in a traffic jam which might be safer than filtering up in an alternative environment.  By simply looking at collisions, fatalities, or whatever, we often forget that these things are an imperfect proxy for the true object of interest: getting around quickly and conveniently with a low amount of risk and effort.  My point is that as advocates, we need to mentally process the statistics and research into something more appropriate for our goals.

Two, the concerns that AASHTO and many traffic engineers have with cycle tracks ( 12)  and similar side paths that have a theoretical and empirical base.  And there are certainly lots of anecdotes from experienced cyclists -- not necessarily VC -- that echo the concerns.  So when we read that there is research concluding the cycle tracks makes cycling safer in an all-things-equal context -- for contradictory evidence read this paper on Copenhagen -- it deserves a lot of skepticism and/or it needs further consideration with respect to my first point regardless of one's beliefs about the world.  Given the strong feelings among certain groups, I want to emphasize that bike lanes, wide-outside lanes, cycle tracks or whatever should still be under consideration for some set of circumstances.  Instead, I want to emphasize that one should be much more careful about implementation than the current euphoria over cycle tracks.

Lastly, in the context of building livable neighborhoods, I share many goals with cycling/walking advocates and other "smart growth" people.  But part of smart growth means being good scientists, properly evaluating all evidence that either confirms or rejects our priors, and giving it the appropriate weight when making decisions and choosing what to advocate.  Most importantly, this includes considering much more difficult topics regarding causality and what we don't know.  

P.S.  M. Kary, a statistician living in Montreal, wrote a response to the paper here.  Thanks to John Allen for pointing out the paper and adapting it for the Internet.  


  1. We have a live experiment with cycletracks going on right now in D.C. I've noticed an increase in bike traffic on 15th st., lots of regular folks in regular clothes riding on CaBi bikes or other utility-type bikes. That's just my observation, but it's something that could be measured by DDOT.

    Safety, OTOH, is difficult to measure, at least until people start getting hurt. I foresee problems with northbound cyclists riding on the left side of 15th st. No one (motorists, cyclists, pedestrians) in DC is accustomed to this, and it goes against what we're taught as kids and what I'm currently teaching my kids, i.e., that bikes, like cars, should ride in the right traffic lane.

    In the end, there will be a cost-benefit analysis. If bike traffic has increased significantly without major incidents, then I suspect cycletracks are here to stay. I think that would be a positive development, and I say that as a VC who prefers riding in the traffic lane and who is not looking forward to being yelled at by motorists for riding in their lane rather than in the designated facility.

    1. This is safety?

      Another factor which you didn't mention is travel time. Obeying the signals results in large increases in travel time (especially opposite the one-way direction of the regular travel lanes -- average speed for a fit, regular bicycle user, slightly downhill, 4.5 miles per hour, see

      I'm not saying that 15th Street without the cycle track was an ideal route for cycling, but I am saying that the cycle track has some big problems. Sure, it will look good to cyclists who think they are bing "protected" -- only they aren't.

      I took part in the expedition on 15th street, and here's another cieo where I nearly collided with a pedestrian and had to brake for an oncoming cyclist who came into my lane. Also note the endemic red-light running:

      What is the solution? I'm more in favor of bicycle boulevard treatments, where bicyclists have the use of the entire width of a street which is traffic-calmed with diverters and bicycle-permeable barriers, so it serves only local motor traffic.

  2. A problem with determining safety ex post collisions/crashes is the low reporting and low incidence rate of such collisions and crashes. In the Montreal study, over ten years, 6 cycletracks and 6 parallel streets produced roughly 550 such incidents; although the data collection was only during the "peak season". Nonetheless, the point is that the counts are going to be low such that most of the inferences are going to be model-based.

    In cars, I understand that they were able to get a better grasp of issues by using cameras and counting "close calls". Perhaps that would be a way to discern more about safety.

  3. Thanks for the discussion.

    On the topic of "Vehicular cyclists (VC) tend to ignore the ability for skilled cyclists to use something like a bike lane to pass motor vehicles stuck in a traffic jam which might be safer than filtering up in an alternative environment."

    A vehicle operator (cyclist) passing another vehicle operator (motorist) on the right violates the basic overtaking rule of the road. Faster vehicles pass on the left. Have you ever had some pass you on the right side, it's very unsettling and for the cyclist a higher risk, i.e. right hook.

    Even is motor traffic is stopped and the bike lane is clear, it's chancy to pass on the right. A cyclist is expecting the motorist to double check in their largest blind zone for a smaller moving object and not cross their path.

    John Allen touches on this topic in comparing a crosswalk and bike box" (amazing bicycle facility of 2009!)

  4. Well, this really isn't a post about passing on the right. I don't recommend doing it in a broad set of situations and I do think that one had better be careful. But "filter up" generally suggests traffic is at a stop and it doesn't mean blow by them like a banshee ... see Moreover, in cases of dense traffic, motorized vehicles are going nowhere fast.

    I think like anything else, we make tradeoffs between alternatives. My casual observation is that some wildly high percentage of cyclists will filter up when there are long queues of motor vehicles going very slow. Doing this with some space on the right is definitely a lot easier and perhaps safer than working your way through cars. They are simply making the choice of getting from A to B faster at some increased risk. Now, I personally think that "chancy" is an overstatement of the risk ratio; but you might have a different scenario in your head. And I'm pretty sure that neither of us have a good scientific measure of it. Consider a cyclist's choices when riding downhill. Is a cyclist riding at 40 mph doing something risky? He/she could always choose to ride at 30 mph instead and it would certainly be safer.

    I still wait at red lights and ride legally; but there are days where I'll get home 15 minutes faster compared to waiting in the regular traffic queue. YMMV.

  5. Despite the cycle tracks on some streets, Montreal drivers do see and are grudggingly (like most drivers) tolerant of cyclists riding on regular roads. It doesnt have to be all one way or another.

    imho at least they see us. I have ridden in places where cyclists do not register on the radar of most drivers.

  6. There's a comprehensive and damning review of the Montreal paper online here: