Traffic on the GSP by Ted Kerwin
In my very occasional series of posts where I give ideas for PhD thesis topics (see the all time hit the students are fighting over, Fractal dimension as measurement of quality) I give you another path to your doctorate. That is if you are in the Operations Research field. Sorry biologists.
I am sure we all have wondered at the situation where you are driving along the interstate and then all of a sudden there is a traffic buildup. You expect that there must be an accident or some event that is causing it, but then, just like it started, it suddenly clears up with no evidence of why it was there. A number of years ago, I remember coming across an article that showed that congestion behaved like a wave (where the height of the wave is the density of the cars). These waves can move through the stream of traffic and long outlive the original cause of their creation. I can’t find the original article, but there has been quite a lot of reasearch in the area. See Density waves in traffic flow for an example.
This morning’s drive into work posed another question in a related light. As I pulled up to the light at the entrance to 199, I had a James City County Police officer in front of me. As we entered the highway, there were not nearly enough cars to prevent free flowing traffic, but we instead stayed all bunched up. Unlike non-law enforcement vehicles, which people are happy to move around and pass to spread out on the road, a police car introduces certain mental restrictions which prevent people from behaving how they naturally would. We are uncomfortable passing a police officer or driving very much faster at all even if we are past the point in the road where the police car is located. This results in a “blob” of traffic that is all stuck together that would not exist otherwise.
So, my topic for a budding Operations Research student is this. How do you model a traffic scenario where certain vehicles introduce more stringent constraints on the vehicles around them that are not purely limited to their physical place on the road?
Go on, you can thank me later.
Update: There is a good comment thread starting over at Hacker News here talking about how this could be used in traffic management. Interesting.
8 Comments
I’m pretty sure this the article you remember:
http://amasci.com/amateur/traffic/traffic1.html
The police have known about this for years, of course, and I’ve heard them claim that’s why they often drive 75 mph in a 65 mph zone: They don’t want to back up traffic.
X: Yup, that’s it! Thanks.
Eric: I agree, I’m sure they know about it. I would say most people know about it in an empirical manner. I’m curious to see what could be learned by formally modeling it.
You might be interested in this too http://www.youtube.com/watch?v=Suugn-p5C1M
After reading this, a question that comes up to my mind is if cops can manipulate traffic lights for tickets…
You might be interested in this book:
http://www.amazon.com/Traffic-Drive-What-Says-About/dp/0307277194/ref=sr_1_1?s=books&ie=UTF8&qid=1332425669&sr=1-1
Seems like in Belgium we’re not as conscious about that fact. When a cop car is doing 120 kph on a highway (where the limit is 120 kph), people overtake it at 125-130 kph and go on their merry way.
Back in the late ’70s, I read what was purported to be a true story, which reflects this situation well. Went something like this:
A guy in a hurry to get home from work in a smaller community came upon a bunch of cars moving slower than the speed limit, due to a police car up front. After a mile or so, the police car slowed down even more, an everyone else did the same, not wanting to pass the police car.
After a couple of minutes of this, the guy decided to enough was enough. He sped up to the speed limit, and passed all the cars–including the police car.
Immediately the police car’s lights and siren were activated, and both the guy and police pulled off to the side of the road. The other drivers laughed at the guy for being so foolish as they drove past.
The officer approached the guy’s car with a brown bag in hand. The guy opened his window to ask why he was pulled over for going the speed limit, and the officer said: “My wife had asked me to pick up some ice cream before I came home. I’d just picked some up a few minutes ago when I got a radio call needing me to pull another shift. I didn’t want the ice cream to melt, so I decided to drive real slow, and give it to the first person brave enough to pass me. Enjoy.”