Well, the truck was definitely real, not an artists concept sketch. An image gallery at the Nevada Department of Transportation website shows the following picture:
The caption simply labels it as an 'Early Department of Highways truck."
Many other interesting images in this gallery, Historical Image Gallery 1917-1939, which gives us a date range, but no ...
It's probably not possible to know how the specific apparatus in that photo works, but it appears that several inventors were trying to automate traffic counting at the time using sophisticated Rube Goldberg types of machines from the pre-transistor age.
You can see some (all?) of them listed in this patent. I briefly go through some in what follows. The ...
The device pictured is likely the product of IBM. Research brings up several books, which unfortunately only offer snippet views, but that is enough for a lead:
Special Report - Page 105
National Research Council (U.S.). Highway Research Board - 1960 -
Snippet view - More editions Recording ...