Our business is strictly on-site - we do not have a store-front. Therefore, EVERYTHING we do has travel involved. Over the years we've tried a number of ways to account for travel.
1. We tried billing for travel based on time, but traffic conditions often skewed the results. Plus, it wasn't always clear how to charge if the previous call was just a short distance, whereas if we were coming from our office it would be a longer trip.
2. We also tried billing for travel based on a rate-per-mile. But that had similar problems as #1. In addition, this method was more intimately tied to the cost of fuel (which, around here, changes on a daily basis!) and frequently had to be changed.
Also, we found that no matter how it was calculated, a highly variable travel charge tended to cause new customers to shop around for techs based strictly on location.
We now use a simple travel charge based on the distance from our office to the client (which we call a "trip charge", a common term around here also used by other businesses, e.g. plumbers):
$30 if in the city, $40 for outside the city and within 30 miles, and $50 for anything farther away. Each service call has the appropriate trip charge regardless of the previous call's location.
We make more money on some some travel than others, but it all works out in the end. And customers seem happy with it.