Do you know when travel rates are the least expensive? By air or by road?

It depends on where you live and where do you want to go. Normally after New year (after Jan 10), before Easter( March), before summer holiday (before Jun 25), after first week of September, before Christmas. The air fare will be cheaper. But the price will be affected by local holiday like if you go to China around Chinese New Year (end of Jan or beginning of Feb).