Compare the letter "s" in both signs. The one of the left is slimmer than the one to the right. But it is not only the "s" it is also the "u" and the "a". This led me think, that the one on the left is written in a "condensed" style (by the way, I'm not sure if "style" is the right word.)
By itself, this is no more than a nifty fact. Here is why this matters. To fit in the space constraints, the design decided to change the style rather than to change the size. Have you had that same problem when designing slides? Even more, reducing the size would be dangerous for drivers who might reduce the speed and concentrate more on the sign than in driving. Just like in a talk, where the audience should concentrate on the speaker and not the slide.
Here is another example of a possible combination with a condensed type:
A warning note: Not all typefaces have a condensed style, thankfully, Roboto and Ubuntu do.