Driving Distance Puzzler

This is another puzzler stolen from Car Talk (which I've been listening to on Podcast now):

A man and his wife decided to go to a bed and breakfast over the weekend. They got into the car and the wife drove for 40 miles. They then pulled over at a rest stop and the husband drove for the rest of the way to the B&B. On the drive back home, the husband drove for 50 miles and then asked his wife to drive the rest of the way home.

The question is, how many more miles did the husband drive than the wife over the course of the whole trip? (You do not need to know the total distance driven to solve this problem)

(highlight to see answer below):

A: 20 miles.

Set the total trip distance to any number and solve. The difference will always be 20 miles. Thus, if d=50, then the husband drove 60 miles while the wife drove 40 miles. If d=100, then the husband drove 110 miles while the wife drove 90 miles.