Capital letters and abbreviations help slow readers and tailgaters
With road signage, visibility at night and in crappy weather conditions is the main priority. But using capital letters is also a sort of cognitive hack that helps you read faster. Back in the day, when cars were evolving out of the horseless carriage phase, lowercase letters were strictly banned on road signs. Eventually, scientists figured out that the mind processes information faster if there are just a few capitalized letters interspersed for visual cues... and it's also a lot less, uh, aggressive.
A fairly long list of allowed and disallowed abbreviations also helps slow readers and anyone whose vision is blocked by traffic (say, if you're stuck behind an 18-wheeler). From the ubiquitous "XING" (crossing) to the understandably confusing and now-prohibited "PARK," which meant Parking, but was frequently misinterpreted as... "park." God only knows why.