Big changes between now and the 1970s. But a couple of rules to be aware of. First is you can be on on-ramps but no further. Hitching on interstates is forbidden except for in, I believe, Minnesota. Good chance of getting busted if you get caught on the interstate. Truckers used to be good for rides, but a bunch of rules has pretty much killed that off. And many trucks in fleets have GPS so they can tell when the truck stops. Manchester, NH used to give a free haircut to any longhair caught hitching in their town.
Depending upon the mood of the cop, it's not unusual for a cop to stop and run the ID of a hitchhiker, especially if they look underage. Great way to find people with outstanding warrants. On the flip side, cops can be good for a ride, and will often take you to a safer location to hitch, again, depending upon mood.
In its heyday, it was possible to cross the country quicker hitching than you could drive it, because of sleeping times.
You're never sure what you're going to get for a driver (or that matter a passenger -- I pick up hitchhikers when I see them -- They're often entertaining, and I figure they need the help. One of the most entertaining was a preacher serving his ministry by hitching the country.). There are lots of stories about the dangers, but no one I knew got into trouble that couldn't be handled. A couple of guys I know probably legitimately had Penthouse letter experiences.
A new expensive car will very rarely stop. Older cars and pickups tend to be the best. Never go with three or four dudes, and if you're a girl, two dudes is one too many. If you're a girl, you dress down a bit. You don't want to look too sexy. Guys, on the other hand, can't be too grungy.
Location for trying to get pickup is important. Cars need to be going slowly, and accelerating. Good shoulder for them to pull off in, and lighting if at night. The better drivers will drop you in good spots as many of the drivers who pick up hitchhikers are aware of the problems. You want a balance between too much and too little traffic.
A grandmother and grandson picking up a hitchhiker would be very unusual. However, a young looking girl is possible.
Young runaways often try to avoid hitchhiking these days. Too many chances of getting stopped by the police. It's better to find someone going your way and getting a ride with them. There are places in most cities where kids hang out and you can hook up for a ride. It's not uncommon to trade a blowjob for a ride. Many girl runaways are sexual abuse victims and a blowjob isn't as bad as it would seem to most people. That's why many of them end up as prostitutes.
Runaways are usually victims of physical, sexual, or mental abuse or have a substance abuse problem or a spur of the moment dumb decision. The situation has become unable to be survived any longer and running seems to be the only way out. Many runaways these days are rather damaged individuals. The runaways I've had to deal with are hard kids to find a solution for. They've learned that the best way to solve a problem is to runaway.
Back in the day, one approach to get a ride was thumb out with one hand and a bottle of a (cheap) Dago red wine in the other. Of course, that didn't say much for the condition of the driver by the time you got to where you were going, but drinking and driving wasn't as bad as it is now.
Jim Clark-Dawe