As a club, we use the following conventions
At the start of any turn a direction dice (We use a GW one) is rolled together with a D6/D10* for each civilian. If a bullseye is rolled, the civilian stops where they are, otherwise they move in the direction rolled for a number of inches directed by the D6/D10, stopping at any terrain or figure they encounter. This can cause real problems if they block Line of Sight (Unless you are a member of the Totenkopf!).
Some common sense does follow, for example civilians don't willingly run into hay threshers or jump into acid pits! They will normally stop before or dodge moving traffic, though if they go over 6" (if using a D10), we deem them to be running in panic so they are fair game.
A more detailed method is used by Westwind Productions in their Ripper scenario (which is available for free on their website).
*Usually D6 on a 3x3 or smaller board, D10 on a larger one.