Why shouldn't that cause an issue? If you have 1,000 cars all making the same trip from the suburbs to downtown, and 90% of the trip is on a 4 lane road, but the last 10% is on a 2 lane road, then the 2 lanes are your bottleneck, and assuming the speed limit is constant, the total trip time of all 1,000 cars will be the same as if the entire trip was on a 2 lane road.

Obviously, real road systems are more complicated, with more cars merging onto the main road along the way, others exiting at different times, etc. But I think the general idea holds true: when thousands of cars are all going from suburban areas into a downtown area, there will inevitably be a bottleneck close to the downtown area.