Even if they had that data, how do they apply it usefully? Let's assume that, even when the odds of a crash are elevated, they're still relatively low. Perhaps

Even if they had that data, how do they apply it usefully? Let's assume that, even when the odds of a crash are elevated, they're still relatively low. Perhaps the chances of a wreck along your route jump from 1% to 10%. If a wreck happens, there's also a lot of variability to how much delay it could add. If it is minor, it could be cleared or moved to the shoulder in 20 minutes. If there is a fatality or a big truck flips over, it could close the whole road for hours. You also don't know how critical it is to the user to arrive on time. Maybe tomorrow the first thing on the agenda is to do an important presentation to management, but the next day the first thing on the agenda is to do some code reviews and catch up on email. So do you need 99% confidence that you'll arrive on time or would 80% confidence be good enough? The answer to that question will have a big effect on how early you have to tell them to leave. (I guess you could just explicitly ask the user, though, which might be nice.)