have you read the itsdf/ansi standard? you can download it for free from the itsdf dot org website, if you agree to not resell the document. I think the part you might want is; "B56.11.6 Evaluation of Visibility from Powered Industrial Trucks"
I agree it is not very specific and not at all specific to the power source (you say you want electric vehicles).
Whenever the standards lack specificity, I always tend think they intend the reader/user of the standards to make great effort to "err on the side of caution".