The way I do random encounters for this is:
I make a list of 6 possible things happening/seen. I will roll 1d6 for that, and then 1d6 to see the suggestion of where it will be.
When an Event used, there will be a new one added to the list. It starts with 6 tier 1 events, then slowly replaced by Tier 2, then Tier 3 and so on. As the tier gets higher, more dangerous and/or weirder things will happen.
The frequency of events depends on movement speed and whether the area have been explored or not. Walking will give 1d6 roll, where on a 6, an event happens. For the truck, it will be a 1d2 roll, where on a 1, one event happens, and on a 2, 2 events happen instead. If traveling over explored areas, the frequency will halve.