While driving on the freeway, look for parking lots behind malls and business bordered by bushes with fencing in between the bushes and the freeway. Find your way to the parking lot, park by the bushes, step through them and post on the fencing. The result should look something like this:
This sign stayed up over three days right next to Interstate 80 and was seen by... well, a hell of a lot of people. This method of posting requires no walking, an absolute minimum of personal exposure and can be done, easily, in less than thirty seconds. Works best in areas with at least a mile before the next exit.