The problem is that such drastic differences in speed create accidents. Someone going 40 on roads where everyone else is going 75 is going to create emergency braking, panic merges, and a bunch of other potential problems.

Jarvis telling me to slow down when the surrounding traffic is going 75 would not be useful, and would (if followed) make me more prone to be hit or cause other accidents.

Instead, what I want Jarvis to tell me is "this car <highlighted in a HUD> is going 30mph slower than you." This is useful, actionable data that a vision or radar system could provide.