For signage just make a full-size mockup on paper and hang it temporarily in place. I've done it many times.

If it isn't huge you may be able to print or Xerox it to size. Or project the image on cardboard (or plywood painted the appropriate color) and use marker pens or paint. It doesn't need to match exactly but should be reasonably close.

The advantage of a full-size mockup is you can view it realistically from different distances, different angles, while walking, while driving, etc.

Also your client can view it along with you and sign off on the sizing.

Yes it's some work, but you don't want a client buying expensive signs and then blaming you they aren't legible.

I believe the author of Wayfinding Sans has devised his own legibility test tool to simulate situations like viewing at great distance or raining or halo effect. You might get some ideas by contacting him. Or, you could just use his Wayfinding Sans.

Herrmann made his own tool for the purpose, which is not available for purchase but is visible in function here:

https://vimeo.com/6376765

I think you can get quite far along the same styles (although in a more cumbersome way) in Photoshop, just using Gaussian blur for distance, outer glow (in layer style, set it to black for black text and darken) for overglow, and a contrast layer for contrast.

More on the developement of Wayfinding Sans: http://ilovetypography.com/2012/04/19/the-design-of-a-signage-typeface/

Although his tool is unavailable, there's a similar feature in Glyphs app. In glyphs view window, there's a preview in the bottom part of the screen, where you can vary different blur intensity and invert colors.

If it's a huge highway sign I can see using simulations. But for ordinary signs, why not make a mockup? Graphic designers do it routinely. And for some large signs, it's possible to mockup small portions of them to test legibility.

Also legibility will vary not only with the weather but also with the viewer. Eyesight gets worse as we age. If it's a sign that's meant to be read by older folks, use some of them in the test.

Blurryness is a convention for “reduced data”. Pixellation is an alternative. Rather like the wave/quantum means of understanding electromagnetic radiation.

Thanks for the suggestions. For those who mentioned Wayfinding Sans and its author, it was after having viewed his Legibility Tool that I was curious to find my own (after seeing his wasn't available to use). I did just come across this though: https://github.com/GitBruno/LegibilityTester. I was looking for ways of testing legibility on a screen, but to actually print it out is of course something that shouldn't be ignored either.

There are many different types of retro-reflective material out there. No digital legibility tester is going to give you results you can trust. The only way to be sure is to make a full-size mock-up using the specified grade of retro-reflective material with the proper overlay film.

Herrmann wrote a post to talk about his design process of Wayfinding.
From his images one can judge that he didn’t “write” his own tool; it was based on Apple’s QuartzComposer.
http://typography.guru/journal/what-makes-letters-legible-r37/

it seems. somewhat difficult to implement
http://www.huz6.com/
http://www.gameark2.com/