# The Physics of the 69-Degree Intersection That Kills Cyclists

WIRED | 1/17/2018 | Rhett Allain
Level 3
Click For Photo: https://media.wired.com/photos/5a5ea23288b9bb357f35be83/191:100/pass/bikehelmet-666782330.jpg

Sometimes when I see an awesome analysis on the internet, I just want to make it more awesomer. Really, this should be everyone's goal on the internet—either make stuff or make it more awesome.

In this case, it's a post from Singletrack (and also covered by Boing Boing) looking at a particular crossroad in the United Kingdom that leads to a large number of accidents between bicycles and cars. One in 2011, one in 2012, and another in 2016—all resulting from the seeming failure of the driver to yield to the cyclist.

#### Problem - Angle - Intersection - Angle - Blind

In short, the problem comes about because of the angle of the intersection (it's not perpendicular) and the angle of the blind spot in the car from its front pillar.

Here's what I want to do. I want to make an animation in python that shows the motion of both the car and the location of the blind spot (they call it a pillar shadow) on the other road. Once I model the motion of the blind spot, I can also find its speed. Even better, after build a model it will be super trivial (which is way easier than trivial) to change the location of the blind spot or the angle of the intersection.

#### Details - Singletack - Post - Roads - Post

Before getting started, I need some details. According to the Singletack post, the two roads cross at 69°. Their post also shows an image of car with its pillar shadow. Using Tracker Video Analysis I can easily measure the angle between the front of the car and both the leading and trailing edge of the shadow (19.4° to 27.1°). Just to be clear, here is a basic diagram of that shadow. Note that this is in the UK, so the drivers are on the wrong side of the car.

Also, the original article assumes that the car will...