What Causes Traffic Jams?

Natalie Josef
September 25, 2012

Where I live, in the San Francisco Bay Area, traffic jams are as common as the fog. A few weeks ago, I drove up to Sebastopol from San Francisco and before I knew it, my leisurely drive had turned into stop-and-go traffic. The traffic was so bad, I had to turn around to head back to the city before I had reached my destination so I wouldn’t be late for another appointment.

When I turned back and started heading the other way, suddenly I was in traffic again. I had seen the roadway on my way up and there had been no traffic—none at all—and now it was crawling along. And just when I was about to seriously freak out, we were moving again.

Long story short, I missed my first appointment and nearly missed my second, all because of phantom traffic jams that seemed to pop up and then dissipate like they had never happened.

What actually causes traffic jams? How do they seemingly materialize like paradoxical oases? Why do they suddenly clear up? What’s going on?

Why Traffic Jams Happen

How can things be moving along at one minute and then suddenly stop the next? This has always perplexed me. It’s the same reason I don’t understand floods. Why doesn’t the water just keep moving? Why don’t the cars just go?

Obviously, an accident can cause a traffic jam. And merges due to construction make sense, too. If you had four lanes and then suddenly only have two, of course traffic will build up.

But what about the traffic that seems to appear out of nowhere?

According to Craig Davis, a physicist at the University of Michigan, the cause of much traffic is selfishness of drivers. According to him, "It's the science of complexity. In large group dynamics, special things happen because each individual is trying to maximize their own benefit."

Davis, who has done research on how to decongest roadways, explains that tunnel vision and delayed reactions of drivers contribute greatly to bringing traffic to a screeching halt.

Let’s say you are driving along when suddenly, a bright red sports car darts in and out of your lane, wanting to pass another car that is moving too slowly for his taste. This makes you hit the brakes, which makes the driver behind you hit her brakes, and the driver behind her, and so on.

Now people are a little ticked and begin to move into other lanes, which had been moving along fine. But because of the new merges, these other lanes now slow down. This process repeats itself over and over until the entire highway has been affected. If the traffic was already dense, things may come to a complete stop. People may exit to avoid the problem and create traffic in town. It’s the proverbial snowball barreling down the hill.

And that guy in the red sports car? He’s probably already at home watching the news of the horrible traffic jam, never imagining his one action led to the gridlock.

Of course, this may all happen without the guy in the sports car. People who tailgate can begin the slam-the-brakes domino effect when they overreact to an event in front of them. It takes people about 3/4 of a second to hit the brakes—using a cell phone makes the response time even slower, which of course, can also make things worse.

The bottom line—it’s the butterfly effect. One little change can cause everything to change.

How You Can Help

Help prevent traffic jams by doing the following (and telling others to, as well):

  • Don’t tailgate 
  • Drive the speed limit 
  • Anticipate—look past the car in front of you when determining how hard the brakes need to be applied 
  • If a merge is ahead, do it quickly—waiting until the last second causes bottlenecks 
  • Leave enough room in front of your car—if cars try to squeeze in there, just let them 
  • When entering the highway from an on-ramp, never go faster or slower than the speed of the traffic already on the highway 
  • When in traffic, try not to stop—just slow down so you can keep moving

Traffic jams are a reality—but they don’t have to be. Be a dreamer and spread the word!

Natalie Josef

About the Author

Natalie Josef is an automotive expert at RepairPal, the leading online source of auto repair resources and estimates. With many ASE Master certified mechanics on staff who have decades of experience, RepairPal knows all the fine points of car repair.

5 User Comments

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By , September 07, 2013
You left out that people looking at the incident on the other side of the road cause most of the problem
By , March 29, 2014
The worst offenders are the "traffic engineers" who plan the roads. These are the morons who design a main traffic artery thru a major city, label it "Circle Drive" & then erect traffic lights every 1-2 blocks. SFBs, every one of them.
By , November 04, 2014 shoulders on the roads in CA.
By , December 10, 2014
I know that route, San Fran to Sebastopol, and it is definitely "If you had four lanes and then suddenly only have two" not to mention the slow pokes in the fast lane that should have let the sports car go by. If you are going slow please stay in the slow lane. Don't pretend your car can go faster than it does. Also that waste of lane called the carpool lane. One free lane that is just sitting there taking up space while it takes 3 hours longer to get home because it's not in use. If I'm not in a hurry I always stay in the slow lane and I always let others pass on back roads. New Hampshire also take note! The worst offenders.
By , September 29, 2015
18-wheelers during rush hours. They take up about 5-6 cars lengths. I wished they were not allowed to be on the roads during rush hour