Safer Motorists or Safer Bicyclists?
Last week, in the first of a three-part series describing my research on bikeway crash risk, I gave relative estimated risks for three types of motorist-caused crashes when bicyclists rode with the flow…
Travel Lane Edge – 61,000 Miles Between Crashes (Highest Risk)
Bike Lane – 75,000 Miles
Sidewalk – 122,000 Miles (Lowest Risk)
… and asked:
Why would the risk be lowest for bicyclists riding on sidewalks?
All crashes occurred at intersections and driveways with no sort of “protection” for the bicyclist. Don’t experienced bicyclists avoid using sidewalks — and sometimes even bike lanes — because riding in the street is supposed to help them avoid such conflicts?
Stay with me while I show you where the data led me as I pondered this question.
There are two key possibilities why these risks are lower: (1) Motorists might be more likely to yield to bike lane and sidewalk cyclists for some reason, and (2) Bike lane and sidewalk cyclists might be better able to avoid a motorist-caused crash.
It’s often argued and assumed that striping a bicycle lane leads motorists to look for and yield to cyclists. But the sidewalk cyclists had lower risk for these motorist-caused crashes than even the bike lane users, though sidewalks are neither designated nor designed for use by cyclists.
When we split up the crashes further, we see that only the risk of drive-outs is higher for bike-lane and sidewalk cyclists. Right-hook and left-cross crashes are less likely:
Right Hook & Left Cross
Travel Lane Edge – 73,000 Miles (Highest Risk)
Bike Lane – 100,000 Miles
Sidewalk – 300,000 Miles (Lowest Risk)
Sidewalk cyclists have the highest risk of drive-out crashes:
Travel Lane Edge – 367,000 Miles (Lowest Risk)
Bike Lane – 292,000 Miles
Sidewalk – 245,000 Miles (Highest Risk)
Would motorists be more likely to yield to bike lane and sidewalk cyclists during overtaking right turns and opposing left turns, but not during drive-outs? I think that this is not more likely.
People Make Mistakes
News Flash: Humans make mistakes, whether they’re walking, bicycling, or driving motor vehicles.
We expect slower automobile and motorcycle drivers to be better able to react more quickly than faster ones to avoid conflicts caused by other motorists.
Why would we assume differently for bicyclists? 
Recall the average speeds we found for cyclists using the three positions. A slower cyclist needs less perception and reaction time and less braking distance:
|Bicyclist Position||Bicyclist Average (Mean) Speed||Bicyclist 85th Percentile Speed||Stopping Distance at 85th Percentile Speed|
|Travel Lane||14.5 MPH||18.4 MPH||104 Feet|
|Bike Lane||11.8 MPH||15.7 MPH||83 Feet|
|Sidewalk||9.3 MPH||12.4 MPH||60 Feet|
Facing an impending motorist-caused crash, the bike-lane user riding at the 85th-percentile speed would need an additional 23 feet of stopping distance compared to the sidewalk rider — about the width of the typical two-lane street. The difference is about the same between the travel lane and the bike lane. The total difference between travel lane and sidewalk is about the width of four traffic lanes, or 44 feet.
85th Percentile Bicyclist Speed and Stopping Distance
With right-hook and left-cross crashes, the motorist is coming from the bicyclist’s left, so the farther right the bicyclist is, the longer it takes for the motorist to reach the bicyclist’s path. This means that the bike lane or sidewalk bicyclist gets more reaction time. With drive-out crashes, the travel lane cyclist gets the most reaction time.
Rather than bikes lanes or sidewalks improving the safety of bicyclists, bicyclists are improving the safety of bike lanes or sidewalks by riding slower.
If you’re going to ride on the sidewalk, bike lane or edge of the travel lane, you must ride slower.
I can’t tell you how slow is slow enough. But if you’re having more close calls with turning and crossing vehicles than you’d like, you need to either slow down, or use lane control. 
Based on the data, each additional mile per hour of bicyclist’s speed increases the risk of a motorist-caused crash about 9 percent.
With lane control, you give yourself much more reaction time for drive-out crashes, you eliminate the right hook crash, and you get more options for avoiding the left cross.
To put it in the simplest of terms: The faster you go, the more important it is for you to control your travel lane.
Sidepaths and Motorist-Caused Crash Risks
We performed the same type of analysis for five sidepaths which have been in place for over ten years. I was curious as to whether they would have better safety performance than a regular sidewalk. They did — about 68 percent better, on average — but it varied widely. Looking closer, I found that three of the paths had few intersections and commercial driveways — 4.6 per mile — while the other two had 11.6 per mile. Low-conflict paths had 64 percent lower motorist-caused crash rates compared to the other two paths; 51 percent lower than for ordinary sidewalks.
|Low-Conflict Sidepaths||High-Conflict Sidepaths||Regular Sidewalks|
|Intersections and Commercial Driveways per Mile||4.6||11.6||10.5|
|Cyclist Miles Between Motorist-Caused Crashes||81,000 (Lowest Risk)||29,000 (Highest Risk)||40,000|
Notice that high-conflict paths had 10 percent more intersections and driveways per mile than the sidewalks, but 38 percent higher motorist-caused crash risk, likely due to the higher bicyclist speeds on the paths (16.3 MPH versus 12.4 MPH for the sidewalks).
If we replace a sidewalk with a sidepath without somehow reducing the turning and crossing conflicts, the risk for the bicyclists will likely increase. This information helps you as a cyclist to decide when a sidepath might be reasonable to use — such as going with the flow on a low-conflict path — and when to avoid one — for example, when going against the flow, or going with the flow fast on a high-conflict path.
Bikeway designers can’t foresee every situation for you, so don’t expect them to.
Next time: Safety in numbers, Vision Zero, and “Preparing the Child.”
 The 1976 Bikecentennial study found that 38% of crashes occurred on downgrades, though the constituted only 15% of the route. This was for all types of crashes.
 This short video explains lane control and the need for it.