usability is about the 5% cases. in broad day light the fonts only compare in a matter of personal taste.
That doesnn't seem to be quite true:
> In 2004, the agency embraced Clearview, based on studies that appeared to demonstrate its superiority, especially in nighttime driving tests.
Granted, mentioning studies generally is different to pointing to specific studies, but still it seems clear that exactly the issues you raise are of concern.
on a clear day you can read font X 10 words in under a second. font Y can be read 12 words in under a second. both are awesome, and a change would not improve anything.
on a night rainy day with little illumination, font X can be read at 7 words per second and font Y can be read at most on 3 words per second. Making font X clearly superior to font Y.