Balance – How often should a party be attacked randomly during his or her journey?

We decided to minimize roaming role plays, which means that in the absence of random encounter, players will reach their destination without talking about what happened on the road.

That said, I still want my players to need horses / cars, and so the more they travel, the more likely they are to be attacked. But what should be the chance of attack? something like 20% each day? 10% every few hours?