The Venturi effect and traffic flow: Why cars go faster on one-lane roads
By Scot Morris
Del MarWith the campaigns on Proposition J being argued back and forth, there has been some confusion on the issue of traffic flow on one-lane vs. two-lane roads.
I hate driving at rush hour. Going north on Highway 101, down Torrey Pines hill toward Del Mar, I clocked myself on this two-lane road, at the glacial pace of just 15 miles per hour. I knew that halfway between the bottom of the hill and light at Carmel Valley Road, the road was going to constrict to one lane, and I resigned myself to slowing even more.
To my surprise, when traffic merged into one lane, my speed doubled to 30 mph!
How can this be? Physics. It’s because of a principle called the Venturi Effect: The velocity of a fluid increases as the cross-sectional area decreases. You see this with your garden hose: Put your thumb over the opening (constricting the flow) and the water speeds up enough to go over the hedge.
The same principle is at work on Highway 101, especially at some distance from the light at Carmel Valley Road. If you add stop signs and stop lights to a road the flow halts and starts up again, so the Venturi effect is less noticeable.
But the fact remains: If, on a two-lane road, traffic flows at a given speed with X cars going by per minute, those same X-cars-per-minute must go twice as fast on a one-lane road!
Strange but true. If you doubt me try it yourself.