If you are looking for a "robust"knife check out Cold Steel's SRK knife.The blade is 3/16" inch thick and six inches long.They are around seventy dollars but you can get them cheaper if you order a factory 2nd.I also agree with the others as to check local laws and carry in the trunk.I do not like it but there are more laws restricting knives than guns.The knife was one of mans first tools. It seems it is harder to carry one than a gun.Before I get shot let me say that I also believe in the right to bear arms.I believe this to include edged tools and weapons.

As for the knife, I would recomend a spyderco Rescue. They are cheap and cut seatbelts very easily. If that knife alarms passengers and cops, then maybe a benchmade rescue hook might answer your question. I would also second or third the recommendaton of a spring loaded punch to break out windows. They arent as big as a hammer, or a s dangerous if you hit a passenger, and will bust out windows in a heartbeat.

Always remember one thing about knifes and seatbelts. Put yourself in a crash situaton and think about your state of mind. If you are in a bad enough wreck to be lcked in your seatbelt, you will probably be a little disoriented. You dont want to be thrashing all over the place with a razor sharp serrated knife in your hand, so keep that in mind when you think about size, etc. I know that some knives are designed with sheepfoot points (the rescue is one) and some even have dull points from the tip back about 3/4 of an inch to prevent you cutting someone else while you cut them out of the car.

I hope I answered your question.

Garrett