I’ve been watching a recent Top Gear episode and learnt the reason behind a little usability rule about capital letters. Top Gear were interviewing Margaret Calvert who worked on the design of Britian’s road signs in the 50’s and she described a little about the typography used in signs.
On road signs you will rarely see words written in only CAPITAL letters. It is easier to recognise a complete word if it starts with a capital, followed by lowercase letters. This is because lower case letters are significantly more different to each other than capitals.