Transportation fiends...Signs and letter scale

[Gringamuyloca]

Is there a governing body or standards which dictate the size of letters on a sign vs highway speed?

Yes... this is a little out of left field, but of all the resources I have when it comes to transportation, this site comes to mind first.

Cheers...

[threestooges]

I'm by no means an expert on it, either in RL or here, but as far as I'm aware, there's no global governing authority on transit sign sizes. I have noticed that faster speeds tend to yield larger signs, especially out on highways like Interstate 10 out in the desert, where the roads are long, straight and the spaces are wide open. I'm not sure though, if that's a product of the location, or the speed. I've noticed other places where similar speeds are marked on noticeable smaller signs. From what I've been able to find though, the MUTCD (Manual on Uniform Traffic Control Devices) proscribes a minimum size for each sign per its location (Table 2B-1; page 3 of that pdf).

I'm sure others around here can give a much more detailed analysis, but I hope this helps.

[Tarkus]

Yes, the MUTCD is the essential resource, at least for US roadways. 

There's usually various engineering reasons involved in the sign and font sizes.

I'm not sure though, if that's a product of the location, or the speed. I've noticed other places where similar speeds are marked on noticeable smaller signs.

If I'm not mistaken, the width of the road also plays a factor.  A two-lane rural highway will generally have smaller signs on it than an interstate, for instance.  It can be similar with surface streets with the same speed as well.  Additionally, if the jurisdiction posting the signs feels that the sign needs more emphasis to stand out, they'll make it larger.

-Alex