[crote]

> You'd literally need to build more roads

Not as much as you'd think, though. At higher speeds you need more distance between individual vehicles, as the vehicles need significantly more time to stop. The rule-of-thumb is that you should keep a three-second gap between vehicles - which if followed would mean speed is completely unrelated to road capacity [0].

The higher speeds are also going to lead to more frequent and nastier incidents, which means more traffic jams. Once the roads get full lower speeds might even result in a higher average throughput!

[0]: https://www.physicsforums.com/insights/analysis-road-capacit...