The Physics of the 69-Degree Intersection That Kills Cyclists in the UK


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Generally after I see an superior evaluation on the web, I simply need to make it extra awesomer. Actually, this must be everybody’s purpose on the web—both make stuff or make it extra superior.

On this case, it is a put up from Singletrack (and likewise lined by Boing Boing) taking a look at a selected crossroad in the UK that results in numerous accidents between bicycles and vehicles. One in 2011, one in 2012, and one other in 2016—all ensuing from the seeming failure of the driving force to yield to the bike owner.

In brief, the issue comes about due to the angle of the intersection (it isn’t perpendicular) and the angle of the blind spot within the automotive from its entrance pillar.

Here is what I need to do. I need to make an animation in python that reveals the movement of each the automotive and the placement of the blind spot (they name it a pillar shadow) on the opposite highway. As soon as I mannequin the movement of the blind spot, I may discover its velocity. Even higher, after construct a mannequin it will likely be tremendous trivial (which is manner simpler than trivial) to vary the placement of the blind spot or the angle of the intersection.

Earlier than getting began, I want some particulars. In accordance with the Singletack put up, the 2 roads cross at 69°. Their put up additionally reveals a picture of automotive with its pillar shadow. Utilizing Tracker Video Evaluation I can simply measure the angle between the entrance of the automotive and each the main and trailing fringe of the shadow (19.four° to 27.1°). Simply to be clear, here’s a primary diagram of that shadow. Word that that is within the UK, so the drivers are on the flawed facet of the automotive.

Additionally, the unique article assumes that the automotive shall be driving at a velocity of 37 mph (unsure the place they bought this however I’ll use the identical worth). Earlier than leaping into python, let me draw an image to assist determine how the calculation will work. Let me begin with simply the vanguard of the pillar shadow and its projection onto the opposite highway.

I’ll begin my mannequin within the easiest way—I am simply going to create the vanguard of the projection for this pillar shadow. However there’s nonetheless some math to do beforehand. Here is the way it will go down. If you need extra particulars, I will attempt to add sufficient feedback within the code so to determine it out.

  • The 2 roads are traces. I can get the equations of those two traces within the type of y = mx + b (slope and intercept). Only for simplicity, each traces will cross by means of the origin (level x = zero, y = zero).
  • Subsequent, discover the placement of the automotive on the primary highway. I want the x and y coordinate of this automotive (this is not troublesome).
  • Discover the equation of the road representing the vanguard of the pillar shadow. That is discovered utilizing the point-slope method for a line The slope of the road is discovered from the angle between the entrance of the automotive and the vanguard of the shadow.
  • Now I want to seek out the intersection between the shadow line equation and the road equation for the second highway. The x and y worth for this intersection is the placement of the shadow projection.
  • Actually, that is it. The one factor left is to maneuver the automotive somewhat bit ahead and repeat the calculation to seek out the subsequent location of the shadow projection.

Sure, it is true. You do not really want a pc program to mannequin the movement of this shadow. In case you like you possibly can discover the rate of the shadow projection with just a few primary math and calculus—I identical to this manner higher.

Now for the primary mannequin. Right here is the animation of the vanguard of the projection. Click on the “play” to run the code and the “pencil” to see or edit the code (don’t be concerned, your edits will not break something).

Instantly you need to be capable to discover that the projection of the shadow on the highway strikes slower than the precise automotive—however don’t be concerned, we’ll get to the speeds quickly. Let me make yet another modification. The next is similar calculation besides that it reveals each the vanguard and the trailing fringe of the pillar shadow.

Right here you’ll be able to see that because the automotive approaches the intersection, the projection of the pillar shadow onto the highway will get smaller. I assume that must be apparent because the pillar shadow has a single angular width—however nonetheless, it is good to see how that may truly look. Additionally, it will have one necessary affect on bicycle speeds. The bike rider would not must journey on the velocity of the main or trailing shadow edge—the rider simply wants to remain between these two spots with a purpose to be invisible to the driving force (which might be a nasty factor).

I am pretty sure that the main and trailing shadow edges transfer at a relentless velocity—however I am not completely sure. Simply to make sure, I’m going to make a plot of the place alongside the highway for each edges and the automotive (all in their very own dimension). Right here is the code (simply in case) and the plot.

From the slopes of those traces, I can discover the shadow edge speeds. I get values of 5.50 m/s and seven.58 m/s (12.three mph and mph). That’s clearly within the vary of potential speeds for a human on a bicycle.

However now that you’ve code to calculate the velocity of the pillar shadow, you should utilize this identical factor for different intersections. What if it is a 90 diploma intersection? What if the automotive is shifting quicker? What when you have a much bigger angle for the pillar shadow? All of those questions are fairly simple to reply by simply altering some numbers within the code. And sure, I already identified that you are able to do this identical calculation on paper—the python stuff is simply enjoyable (and also you get an animation).

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