Maintenance vehicles and unauthorized vehicles on a partially loaded blast pattern can be a serious safety issue.
As a blaster I have observed this problem first hand. We would leave a blast pattern at the end of the shift loaded to within a "safe distance" from the drill and come back the next morning to discover vehicle tracks over the cuttings pile of a bore hole caving it in. If the bore hole was then too short to load and happened to be within the safety perimeter of loaded holes we had a problem. Should we eliminate loading the hole which would create "high bottom" or try to sneak the drill in to clean out the hole? I won't try to give an answer to the problem because situations vary.
In a worst case scenario the vehicle has run over a loaded hole. This could cause damage to the down lines or worse yet the hole could have detonated. We have tried to educate maintenance personnel of this possibility which is caused by a phenomenon called "snap and shoot". In theory this has happened with shock tube down lines and an accident on a pattern was attributed to this.
Blast pattern security is a problem because maintenance on the drill is necessary and due to production demands the drill operator doesn't like to take the time to move the drill off the pattern and sometimes a breakdown prevents the drill from being moved.
I guess the answer is common sense and stronger safety rules.

by Jim Keenan, retired blaster