In the setup described here, all the roads have infinite throughput. There's no level of traffic at which they stop working altogether; you can push as many car

In the setup described here, all the roads have infinite throughput. There's no level of traffic at which they stop working altogether; you can push as many cars per unit time through either network, and that many cars per unit time will come out the other side. But, as with many kinds of system, you may pay a heavy price in latency if you insist on getting lots of bandwidth. But there's something right in what you say. When you add the extra road you make travel time worse at low usage and better at high usage. Suppose your network has an upper path with cost for x cars 1 on the left and x on the right, and a lower path with cost x on the left and 1 on the right, and the optional vertical road is free at any capacity. Then, if I've done the calculations right, average travel time with t cars is 1+t/2 without the extra road, and min(2,2t) with. So at t=1, which is the case usually used to demonstrate the paradox, the extra road increases the travel time from 1.5 to 2, but beyond that point increased traffic is free for the enlarged network but not for the original one. So, e.g., if t=10 then the extra road makes things 3x faster. The crossover point is at t=2, where both networks have average travel time 2. So the extra road is a win if you expect t >= 2, and a loss if you expect t <= 2. (If we measure throughput as "max traffic with such-and-such a limit to average travel time", then if the limit is <= 2 then both have finite throughput and the one with the extra road is worse, but if the limit is >= 2 then the network with the extra road has infinite bandwidth.)