Ask the Experts: Søren Underlien Jensen and Dr. Lon D. Roberts, PhD.
July 8, 2009 § 16 Comments
A study released in Copenhagen titled “Road safety and perceived risk of cycle facilities in Copenhagen”, is sometimes referenced by local Vehicular Cyclists as proof that cycling infrastructure poses a greater safety risk, though they typically avoid the paper’s conclusion which clearly advocates in favor of infrastructure. Noting this confusion, a BFOC commenter posted the following request:
I’d appreciate a thoughtful post about this study, which is about the effects of bike infrastructure in Copenhagen. I have doubts about the methodology, it not being explained real well, but the conclusions include:
Cycle tracks increase cycling 18-20%
Cycle tracks increase accidents 9-10%
Cycle lanes were less effective at increasing cycling and it was unclear if they raised accidents more than cycle tracks
The paper did not discuss to what extent (if any) all this infrastructure caused cyclists to lose road riding rights they enjoyed previously.
To clarify the paper, we contacted the study’s Danish author, Soren Jensen, for a breakdown, and asked if bicycle facilities would benefit Dallas. We also communicated with Dr. Lon D. Roberts, author of SPC (Statistical Process Control) for Right Brain Thinkers and Professor of the course Gleaning Facts from Figures to provide a better understanding due to confusing concepts in statistical analysis. Before getting started, some definitions are in order…a cycle track (pictured above) is simply a dedicated bike lane, which is physically separated from car traffic, while a bike lane is simply a painted line on a street that has no physical barrier between auto lanes.
Below is our conversations, starting with Mr. Jensen:
The cycle tracks (kerb between drive lane and cycle track, and kerb between sidewalk and cycle track) increase cycling by 18-20%, whereas cycle lanes (only a 30 cm wide white marking to drive lane) increase cycling by 5-7%. These figures have been found for streets in Copenhagen. I do not know, what the figures would be in Dallas, but because cycle facilities are seldom in Texas, I believe the figures would be higher (much higher).
The cycle tracks have resulted in an increase of 9-10% in both accidents and injuries. The study accounts for confounding factors (general safety trends, changes in traffic volumes and regression-to-the-mean), i.e. the stated safety effect is truly the result of constructing cycle tracts. There are several reasons for the increase in accidents, however, the most dominating one is that construction of cycle tracks on main roads often leads to a parking ban on these roads, which then leads to many automobiles being driven onto sidestreets and being parked there – and this leads to many more accidents at intersections between main roads and side streets. If parking is not banned on the main road then there most often is no change in safety when cycle tracks are being constructed. The cycle lanes have resulted in an increase of 5-15% in both accidents and injuries – i.e. practically the same as cycle tracks. Both cycle lanes and cycle tracks are most often about 2.0 metres wide in Copenhagen. All investigated cycle tracks and cycle lanes are one-way, i.e. there is one cycle track/lane in each side of the road.
The Traffic Act in Denmark clearly states that if a road has cycle tracks or lanes, then the bicyclists must use them, i.e. they must not ride on drive lanes or sidewalks – only children up to 6 years old may ride on sidewalks.
In the past 5-8 years, the construction of cycle tracks have often been a part of a more comprehensive “campaign” in order to get more people cycling in many communities in Denmark. The size of the possible spil-over or synergy effect on cycle traffic volumes, which might come from mixing new cycle facilities with campaigning, is unknown – and is also may have implication on safety. But I do not know if an increase in bicycle traffic in Dallas will lead to better or worse safety overall (total number of road deaths etc.) – but I do know that it will lead to better safety for the bicyclists.
Søren Underlien Jensen
Could you clarify one part for me? You stated that
“The cycle lanes have resulted in an increase of 5-15% in both accidents and injuries “
but ended by stating “I do know that it will lead to better safety for the bicyclists.”
If you’re seeing an overall increase in accidents, would this not lead to less safety for bicyclists?
There exists clear relationships between the traffic density and traffic safety for all travel modes (from walking to aviation – probably even space shuttles). This relationship says – higher density, better safety. An example is P L Jacobsens paper http://injuryprevention.bmj.com/cgi/content/abstract/9/3/205
Søren Underlien Jensen
Got that? I too was a bit unclear. So, I contacted Dr. Lon D. Roberts, author of Statistical Process Control for Right Brain Thinkers to explain in greater detail.
Like with most studies involving statistical analysis, one must be careful as a case can be made for and against a particular position, depending on how the users of the data choose to “cherry pick” the results. Looking at the big picture, based on Soren’s study, his comments to you, and the other study he referenced, there is a simple syllogism to use that can be backed up by the numbers and percentages that shows favor for the development of bicycle facilities.
* The likelihood an individual bicyclist will experience an accident goes down as the number of bicycle riders go up.
* So, if an increase in bicycle ridership leads to a lower likelihood that a particular individual will have an accident, how do we increase bicycle ridership? According to Soren’s study, the way to do this is to construct more cycle tracks and cycle lanes.
* Conclusion: construct more cycle lanes and tracks.
In formal logic this would be expressed as follows:
Major Premise: The per capita likelihood of a bike accident decreases as the number of bike riders increases.
Minor Premise: The number of bike riders increases as the number of bike lanes and bike tracks increases.
Conclusion: Increasing the number of bike lanes reduces the per capita likelihood of a bike accident.
Keep in mind that “inferences” can be drawn from statistics, but this is not the same as saying the statistics “prove” a particular conclusion. (Every statistician will tell you that you can’t “prove” anything with statistics.)
While the bike lanes do not seem to have an effect one way or the other, if someone tried to use Soren’s study to “prove” that an increase in cycle tracks increased accidents by 9%, they’d be guilty of cherry picking the numbers. The accident rate may have increased by 9%, but the number of bicyclists increased by 18-20%. (This is consistent with the Major Premise cited above.)
Using Soren’s percentages, here’s an example starting with the assumption that 10 bicyclists out of 10,000 will experience an accident over a certain period of time if there are no bike tracks:
1. On an individual basis, there’s a 10 out of 10,000 (or 0.1%) chance that an individual biker will experience an accident if there are no bike tracks
2. When the bike tracks were added, the accident rate increased by 9%. In other words, if there are 10 accidents without the tracks, the number of accidents increases 10.9 (or approximately 11). On the other hand, the number of bike riders increased by 18%, from 10,000 to 11,800. Therefore, on an individual basis the likelihood of an accident with the tracks added is now 11 out of 11,800, or 0.09%, as opposed to 0.1% without the lanes/tracks.
Even if you assume there are 1000 bike accidents out of 10,000 without the lanes/tracks, the likelihood on an individual level is 9% with the lanes/tracks rather than 10% without the lanes/tracks.
So to sum up, individual accident rates dropped when bicycle infrastructure was added, and taking that a step further, Soren’s follow-up correspondence recommends that if Dallas added cycle tracks, ridership would be “much higher”, and “that it will lead to better safety for the bicyclists.” He even recommends maintaining parking on streets to further decrease accident rates.
Special thanks to Søren Underlien Jensen, Dr. Lon D. Roberts, and our commenter, Steve A., for requesting the post.