The key with Mexico is that all traffic is unpredictable. The US gets into an unfortunate "local maxima" where the car traffic is relatively predictable so anything that deviates is much more dangerous.

Whereas free-for-alls like India, et al, are predictable in their unpredictability - you can't rest for a moment because everything is chaos.

There's also a tipping point when you have enough crowding that everything slows down.