It turned out that the fire wasn't deliberately set. It looked like it to passing motorists, I guess, but the fire investigators determined that it was carbon emissions from a car that contained solid pieces that ignited the brush at the side of the road (because it is so very dry now). They haven't yet found the car responsible. They are warning residents of fire-prone areas to make sure their cars are in good working order.
I recall hearing about other fires being set by catalytic converters in tall brush when people pulled off road to park.
This fire season is just not going to allow any margins of error. We all have to be more careful.