Damn this traffic jam, time I get home my supper'll be cold, damn this traffic jam.
Well I left my job about 5 o'clock, it took fifteen minutes go three blocks,
Just in time to stand in line with a freeway looking like a parking lot.
-- James Taylor
Most of my research has examined how the performance of the average driver is altered when they talk on a cell phone. Our focus has primarily been on the implications for traffic safety, but here I want to focus on a related topic, the efficiency (or flow) of traffic.
We know that one characteristic of a driver using a cell phone is that they do not drive with the normal flow of traffic. For example, I'm sure you have noticed the driver stopped at a green light chatting away on their cell phone. You might even have had to gently tap on your horn to get them to go on their way.
Using our simulator, we have found that the reaction time of a driver on a cell phone is slowed significantly and that when they hit the brakes in traffic that it takes them about twice as long to return to highway speed as when they are not on the phone.
I teamed up with a group of traffic engineers to determine what happens when you add more and more of these distracted drivers on the road during rush hour. We used the software that the traffic engineers use to design highways and model the flow of traffic. The simulations allow us to determine the number of vehicles per lane per hour that travel past a particular point on the highway.
As we added more and more drivers on the phone (randomly distributed throughout the highway), we found that the traffic flow of all vehicles declined. In fact, we found that when the number of drivers talking on a cell phone reached 10% (the current level estimated by the National Highway Transportation Agency) the commute time increased by 10%!
Here is another way to say this. Each driver talking on a cell phone slows the drive for everyone behind them. The cumulative effect of a bunch of drivers talking on their cell phone is that the total commute time for everyone is lengthened. The estimates indicate that up to 10% of your daily commute in rush hour is caused by other drivers talking on their cell phone.
The societal costs for having the commute time for all drivers lengthened by 10% are huge. So too are the increases in fuel consumption and degradation of air quality associated with longer commutes. These are just some of the hidden costs associated with using a cell phone while driving. I'll address the related health costs associated with rush hour traffic in another post.
So, if it feels like your commute has gotten a little longer, that is probably because it has.