For every four days or part thereof of travel across the Wasteland, one tests on:

(Luck + (Survival : 25)) using a D10.

If the test succeeds, there is a positive random encounter. If it fails, a negative one.

Caravans test on a fixed value of 5.
Groups of several characters use the best value within their group.

To determine what kind of positive or negative encounter happens, one can roll a D20 and go by the list below.

Negative encounters

1: Deathclaws

2: Rats

3: Wolves or wild dogs

4: Radscorpions

5: Giant mantises

6 – 9: Feral ghouls

10 – 14: A band of Raiders or Slavers (not in NCR territory, here one would meet a patrolling group of Rangers).

17: Spore plants

18: Sand storm

19: Very strong rain

20: incredible heat