Roads cause traffic
Yeah, it actually does need to be said. Folks with an investment in expanding highway infrastructure like to act as though the only way to relieve congestion is to build more roads for those cars to ride on. But a soon-to-be-published study shows that traffic expands to fill the space allotted. More roads don't mean more room — they mean more cars.
There are a few reasons for this, but mainly it's that people drive more when it's easier. It's not that we need to have a certain number of cars on the road, and that number just happens to be a little more than what our current highway system can support. It's that people basically drive until the available roads are hideously congested, then stop. Building more roads to relieve congestion is like suggesting that a blood pressure patient should keep eating fried chicken (or whatever), but surgically add a few yards of arteries. It'll work for a little while, but those are just gonna get clogged too. And in the meantime, you're walking around with extra arteries draped all over you. Unsightly!
Study: Building Roads to Cure Congestion Is an Exercise in Futility, Streetsblog DC.