This is my opinion: I think it depends on the dog. I have one for mine because fencing is not an option for us. I have to add that we live in in rural VT and have lots of tempting critters all around us (moose, bobcats, fishers, porcupines, deer, etc.) and the odds are less likely to be hit by a car, only because it isn't as populated as other areas, which we considered before purchasing one.
We did all of the training with the flags and on leash training. We set it to the different levels, as instructed. We kept increasing it until she respected it. She bolted through once and never again. I will say that it is on the next to highest level and if she runs through it, she will be a tiny little Basenji mushroom cloud!
Seriously, she is conditioned now that she actually has gone out without her collar (or with dead batteries) and goes nowhere near the boundary lines. Not that I would recommend that at all, but it has happened.
If you do get one, make sure it has a lightening protection module (some are separate, others inside the unit) as it took out our TV in a thunderstorm. It's a very inexpensive add on.
I'm sure you'll take all the info. given on this thread and make the best decsion that suits your situation.