1.) Estimating driving times:
Prediction = Pessimistic guess + 4(mean estimate) + (optimistic guess) /6
So for me, my pessimistic guess is twice the Yahoo! Maps time estimate, the mean estimate is the Yahoo! Maps estimate, and the optimistic guess is also the Yahoo! Maps estimate.
So I take the Yahoo! Maps estimate and add a sixth to it and that's my estimate. Works pretty well.
Another thing I do is compare how early I can get there and not be too early (usually the check-in time at the hotel), and when I actually want to get there and use the Yahoo! Maps estimate to split the difference.
2.) Taking driving breaks:
I think I adapted this from Geico magazine. It's called "Halves and Thirds." This trick helps you when you're tired to compensate for bad judgement about when you should stop.
Use Yahoo! Maps to estimate your total mileage. 1) Reset your trip odometer. Prepare to drive a third of the distance without stopping. 2) Try to drive a third of the distance without stopping. If you get halfway without stopping, take a break whether you feel like it or not. 2) Take a break. Walk and stretch like you're warming down after exercising, because you are. 3) Compute how many miles you have left and write down a third and a half of that distance. Reset your odometer and try for another third or half. Once you stop after driving halfway, either stop one more time after cutting it in half or finish the trip.
So if you have 1200 miles to go, your first leg is 400-600 miles. Say you stop at mile 480. Reset your odometer. 1200 - 480 = 720 miles, so your next leg is 320 miles or 360 miles. Say you stop at mile 360. You've made a half leg, so your next break is either mile 180 or mile 360 (your destination), 'cos you've already driven a 360-mile leg today.
This cutting the mileage into legs of minimum thirds and maximum halves spreads out your breaks evenly. It makes you take breaks just a little bit more often the farther you drive, and eliminates the risk of highway-addled bad judgment about when to stop.