The capacity of the road as being the number of cars passing by per second. Which is the number of lines times the rate at which cars are traveling, divided by the distance between cars in a lane. Now compare:
Smooth: Going 60 mph and leaving 2 car lengths of space, so the distance from front bumper to front bumper is 3 car lengths.
Congested: Going 30 mph and leaving 1 car length of space, so the distance from front bumper to front bumper is 2 car lengths.
In the former road you go 2x as fast and need only 1.5x as much space, so the former road has a third as much capacity as the latter. So if cars are moving along a highway already operating at capacity into a congested area, cars are arriving at the transition faster than they are leaving. Thanks to the law of conservation of cars, excess cars must accumulate at that transition, and since they have nowhere to go, they wind up parked in a traffic jam. And as long as the road feeding it is at capacity, that traffic jam grows.
From the point of view of the driver, this is what it looks like. You're sailing along in smooth fast traffic, and then hit a sudden traffic jam with no warning. After sitting there for a long time the road starts moving again, but goes much slower.
This is common experience in Los Angeles traffic. And a significant fraction of accidents happen at that sudden transition from smooth sailing at 60 mph to a dead stop. (In fact my wife got rear-ended in that exact scenario a year and a half ago on the 405 near Los Angeles airport.)