carry wheel chocks!!!!
IMHO, very few of the jacks in cars make me feel secure about even changing a tire. I like bottle jacks (one came in my old Toyota Land Cruiser) under heavy suspension components.
When you use one of these point jacks, chock your wheels, no matter how flat the ground seems. I carried around a couple 6" pieces of 2x4, until I found some metal folding chocks at Walmart for $5-6. They lie flat and carry easily, and they work. My wagon got a flat on the ramp between 87S and the GW bridge in NYC. At night, no less, on the curviest section. Major incline! The chocks made the difference between getting home 20 mins later and being on a dark and dangerous ramp for a few hours waiting for a tow truck. And most of the cars weren't expecting to see a disabled car as they came around the corner towards me at 40+mph.
I have had a few jacks swaying under the weight of a car (VW Beetle, Grand Wagoneer, lots of Japanese cars). Not happy! Now that I think about it, time to get a few chocks for the wife's new car!
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K