I am working so don't have a lot of time to do exhaustive research but I did find one outdoor camera that claimed to only draw 1/2 watt and could be serviced with 1.5 sq ft of solar panel.
They actually said 1.2 but in the case of solar I am very strongly inclined to over engineer capacity. panels get dirty, birds poop on them, the sun can be hidden by clouds for days on end, and as they age the slowly lose some of their initial capacity. Given this will be hard for us to service (unless someone who lives local is willing and able) better design for bullet proof reliability. I imagine panels similar to those you see on road side signs would be adequate.
It also depends on what sort of bells and whistles folks are willing to fund for the primary install. If it is a panning camera, the local government or state highway might be willing to chip in a few $$ if they can use it to pan around or piggly back another camera to view the highway if they have accidents or other issues they want to watch.
