I wouldn't drive across it too often.
A drain field in heavy clay soil is bad enough.
The clay keeps the soil from absorbing the water.
Driving over it packs it further and makes it worse.

That being said, at my last house I had an access road
across the drain "field". The field was one long pipe.
Didn't seem to hurt it any but Only drove over once or twice a week.

My house now I drive across the drain field and septic tank
with a 3000 pound ford tractor everytime I mow the yard.
Probably 3 times in two weeks, maybe less.
Hope I'm not hurting anything. Lived here 2 1/2 years so far.

Best advice: don't do it.
Personal experience: hasn't hurt anything yet.
YMMV (your milage may vary) (Disclaimer)

Pooh Bear

Pooh Bear, Thanks for your reply. I have heard that driving on it is bad but don't know if those reports are from actual experiences, or just hear-say. I hope we hear from some experienced responders before you and I both wreck our drainfields.

My hubby installs septics and sand mounds...You shouldnt drive on the fields.a small mowing tractor ok but nothing bigger..