Courtesy Space

I LOVE driving in traffic! No really, I do. And here’s why: I leave Courtesy Space in front of me when I’m driving in traffic. What is Courtesy Space? I’m sure you can guess, but formally what this EXTRA gap between my car and the car in front of me is is an area the assholes impatient of the world feel compelled to commandeer. This space is a gap where folks can merge into when we have to zipper (on-ramp merge). And this space is a cushion which I allow to be squeezed as those in front of me come to a halt while I gradually slow down, keeping those behind me moving, until — quite often — the car before me starts moving again and NEITHER I nor ANYONE BEHIND ME had to STOP!

I call this Courtesy Space. Now, don’t get me wrong; without traffic I’ll gladly drive speed limit + 5mph (on average and on the highways, speed limit or lower on side streets). But in traffic, this is how I drive.

If we all gave each other Courtesy Space THERE WOULD BE NO TRAFFIC PROBLEM. Really.

Here’s the evidence:

MileHourVehicles

https://docs.google.com/spreadsheets/d/12wHZCrd8AqG0Ob9mjbWenNGpfHUe4ruGYVeUcePHXP8/edit?usp=sharing

Now, this is going to hurt a little bit, but I promise, it’ll feel better when I’m done…

  • Let’s take a 1 mile stretch of highway.
  • On that single lane highway we’ll line cars up, end to end.
  • If we estimate a car is 20 feet long (on average: motorcycles, trucks, SUVs,  delivery, etc.) then we can fit 264 vehicles on that 1 mile stretch.
  • Now, let’s pretend this is a traffic situation and we will slowly roll these 264 cars along at 1 mile per hour (We’ve all been there right?)
  • Then, in 1 hour, we will have moved 264 cars 1 mile,
    or: 264 MHVs – Mile Hour Vehicles. (The left most green square above.)
  • This becomes our baseline for comparison.

With me so far? You’re going to feel a slight pinch in this next part.

  • At 1 mph, let’s say this travel mode is “safe” — having a Caution Factor of 1.
  • Caution Factor is simply a multiple of average car length, (in this case 20) minus one car length, that is:
    • CF-1=0 feet
    • CF-2=20 feet
    • CF-3=40 feet
    • CF-10=180 feet, or 9×20.
  • Caution factor can be thought of as Courtesy Space.
  • Now let’s take those same 264 vehicles on our 1 mile stretch and get them moving at 5 mph. Remember — this is traffic and we have drivers driving these cars.
  • At 5mph we *could* get 1320 MHVs, but we can’t possibly allow cars to travel bumper to bumper without problems so the Caution Factor of 1 is red — no good. And, I’m going to figure that even if we increase the factor to 2 (we’ll leave a whole car length in between each car) that is STILL not safe. So 2 Caution Factor is red also — no good.
  • But at a Caution Factor of 3, that is, 3 times the vehicle length (40 feet in front of each car (20 feet for the car remember)) well, THAT is safe. And that’s our second green field with 440 MHVs.
  • We got 440 vehicles to go 1 mile in 1 hour with this configuration (5 mph and 40 feet of Courtesy Space.)
  • That’s a lot more cars and no accidents or road rage.

But we can do better… Let’s skip to the far right.

  • Let’s boost the speed to 60 miles per hour!
  • BUT, let’s boost the Courtesy Space by a factor of 9 (a total Courtesy Space of 160 feet between each car.)
  • 160 feet — that’s TWO TENNIS courts end to end. That’s so much space you could put 8 other cars in there.
  • BUT DON’T! That’s our Courtesy Space — we NEED that space for our buffer zone. So, how many cars can we get to travel 1 mile in 1 hour with this configuration?
  • 1760! That’s more than SIX times as many cars as the 1 mile an hour crawl.
  • BUT, BUT look at all that empty space between cars. LEAVE IT!
  • This is the point. By increasing the space between cars * AT SPEED * we can increase the carrying capacity, the Mile-Hour-Vehicles that a road can carry.

(A minor point: Look at that number right next to that 1760: “29” cars per minute. Think about that: car #1 goes by, one-mississippi two-mississippi, car #2… That’s one car every two seconds. Hmm, could that be the two second driving space rule we heard about way back when we were being taught to drive?)

We don’t need more highways? We don’t need road-rage management. What we need is to realize that highways are not parking lots — we don’t add capacity for standing cars. We need to measure traffic by the metric Mile-Hour-Vehicles, that is, how many cars can we get moving as quickly and as safely as possible. Using this measurement we can see that the best way to cure our traffic ills is through Courtesy Space.

Thanks for your time. Here’s a band-aid and a lollipop.

 

 

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