It’s like if you’re standing in line at a checkout. It doesn’t matter how much area is reserved for people standing in line: if the store can’t support more than, say, 1 person checking out per minute, then when enough people want to buy something, you’re going to stand in line for a while; you’re limited by the slowest part of the process.

Driving in a city is the checkout part; it’s inherently slower than driving on a highway. So you can add as much of a buffer as you want (in the driving example, that’s what the extra lanes do; in the checkout example, it would be adding more space to stand in line), but it can’t possibly address the actual choke point