You would have to be careful with any jack being used when the vehicle is on a slope.
Advice from the Gecko. "Step-by-step guide: . .
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2. If you have a choice, pull as far off the road as possible and on the flattest and firmest ground you see. The more level and solid the ground, the better.
3.Call for help if you are in an emergency lane or center lane on a main street or highway. It is not safe for you to work on the vehicle unless you are well off the road with plenty of room to work. If your only option is a soft surface, do not attempt to jack up your vehicle as this can pose major safety concerns. Call for help."
