The best thing about this scenario is:

1. Reminds me to leave a note as to here I'm going and when I'm returning.
2. Truck? Easy to pack a bit extra - extra food, water, clothes to allow for weather changes, tools, etc.
3. In 98% of cases, I'd wait it out by the broken truck.

Everything but staying put and signalling for help adds risks , and opportunities for decision-making in degraded mental and physical states for which you are not equipped; that will not end well. Being bored and perhaps embarressed while providing an unchallenging rescue.

Route#2 is mentioned to be shorter, but by how much? How well do I know the roads, and how well are the roads marked? Route 1 appears doable, but water is a major concern, with that heat (walking the road at night seems risky), you are going to burn through alot of it very quickly. I would guess (conservatively)progress of 12-18miles per day depending on how easily I took it and how much foraging I would have to do for water.

If it is just a vehicle breakdown then it's a simple no-go decision or at least not go very far, in my mind. The road may have infrequent travellers but as you pointed out, YOU met them on the trip so the chances of them meeting up with another vehicle are quite possible. They have food, shelter and means to keep themselves comfortable for at least a few days. Not a great reason to pick and go if you are not equipped or don't have the ability/knowledge to walk out safely. The environmental conditions are not severe and if they keep themselves dry, hypothermia is not likely. Wait it out, at least for a day, then re-evaluate your situation.

You mentioned they had typical car camping equipment -- without a pack or the knowledge to fashion a reasonable pack, the basic ability to take their supplies with them is pretty limited. Which means they are much more likely to try to walk out with very little supplies and with the hope that they meet up with someone along the way.

2ndly, if it is more than a vehicle breakdown (a medical emergency as well or perhaps a nearby forest fire, etc) then that might prompt the campers to hike it out sooner rather than hunker down. But since you didn't supply that info in your scenario, Option A is still the best bet.

That's my opinion -- however, you never can tell with some people who might over stress and not think rationally about their situation. There will be some overconfident skill lacking people who will attempt to hike out.

We get the same thing here in Saskatchewan. The vehicle breaks down but lets say its in the middle of winter with a blizzard on the way and the farm house you can see is only 1.5 miles away. Smart money says stay and wait out the storm but idiots will pack up and try to hike that short distance unprepared. People have and will die in a significant percentage of those who choose Option B.