Computers frequently have an "auto on/off" function that you may have to set. If this function is turned on (I use an Avocet), the computer will probably only need to be cleared between rides and then it will calculate avg sped based on ride time not total elapsed time. Just remember that the computer will avg in the speed your bike is travelling even when walking it from the door to the street.

I'm unfamiliar with Topeak computers so don't know how they function but my old Cateyes worked the same way.