Mostly yes, but it depends. For fixed-message road signs, mixed-case outperforms upper-case, according to Garvey et al. (1997), Effects of font and capitalization on legibility of guide signs. Transportation Research Record 1605:73. In their introduction, they state that
Forbes et al. (2) conducted what are perhaps the definitive studies of
the difference in legibility between text depicted in all uppercase letters
and that depicted in lowercase with initial capital letters. When
upper- and mixed-case words occupied the same sign area, Forbes and
his colleagues found a significant improvement in reading distance
with the mixed-case words. It must be understood, however, that these
results were obtained with a recognition task. That is, the observers
knew what words they were looking for. In instances in which the text
is not known to the observer, improvements with mixed-case words
are not evident (1,2). Although mixed-case superiority is fairly well
accepted in the traffic engineering community [Markowitz et al. (4)
provided specific information suggesting the use of mixed-case lettering
for conventional road guide signs in 1968], conventional road
guide signs still are being created with all uppercase letters.
Assuming that the authors can summarize prior research, I didn't hunt down the Forbes reference; here's the info for those who want to look at the original source:
Forbes, T. W., K. Moscowitz, and G. Morgan. A Comparison of Lower
Case and Capital Letters for Highway Signs. Proc., 30th Annual Meeting
of the Highway Research Board, Washington, D.C., 1950.
After testing their new font, "Clearview" (which is, to me, indeed easy to read), Garvey et al. find again that
The mixed-case Clearview characters outperformed the all-uppercase
Series D by as much as 14 percent in daytime and 16 percent at night,
as long as the mixed-case font subtended an equivalent sign area. If
the mixed-case font took up less sign space, as with the Clearview
Condensed at 100 percent, there was no difference between mixedcase
and all-uppercase characters. During daytime testing there was
no difference between Series E(M) and any comparably sized
Clearview font (i.e., Clearview and Clearview at 112 percent). At
night, however, with both high-brightness materials, the Clearview
font at 112 percent outperformed the Series E(M) by 16 percent.
Note that things are different for the variable-message road signs (i.e. the ones you may see at road construction sites. There, all uppercase is better, according to Collins and Hall, (1992). Legibility and readability of light refecting matrix variable message road signs, Lighting Research and Technology 24:143.
In general, letters with any regular shaped pixel, a width/height ratio approaching 1.0, an upper case font and a letter separation of two pixels were found to be most effective.