One of the issues must have been learning exactly where to park to have comfortable access to a particular light. Perhaps there was a ceiling window in the vehicle. In the photograph, the workman looks as though he would have liked to have parked about a foot further forward from his present spot. Of course, the slope of the ladder would mean that the perfect spot would change with the height of the light above street level. And then there are allowances that might have to be made for sloping roadways.
I would assume that it was done by driving slowly until the working spot on the ladder appeared to be just short of brushing against the light.
-Marty-