Another solution would be to use a letter following the vowel in each word to indicate that word’s tone. This is not a new idea; the famous Chinese lexicographer Lin Yutang (I think that’s his name) applied this system to either Wade-Giles or Yale romanization in his dictionary, and more recently the system has been used with Hanyu Pinyin by Professor Cornelius Kubler at Williams College in America, whose texts I used when I began studying Chinese.
It goes like this:
first tone, no change: ta (him/her)
second tone, add a y: huiy (to return)
third tone, double the vowel: woo (I/me)
fourth tone, add an h: duih (correct, yes)
There are few guidelines about which vowel to double when the word contains two and so on, but really the system is invariable and simple.
I would advocate this solution, mostly because I think it resolves a fundamental problem with the use of tone marks over each word. The universal failure of sign makers, map makers, and so on, to use those tone marks, is one facet of that problem. To put a name on it: to Westerners - who are the principle users of any romanization system - and especially to English speakers, tone marks over a word are simply not considered an integral part of the word’s spelling. (This is to some extent not true of Spanish and French.) And as such, they are omitted nine times out of ten. Hence they become, effectively, a non-functioning part of Hanyu Pinyin: if they are not used when the words are written, they cannot be used when the words are read, and a vital part of the word’s pronunciation is regularly dropped from its written form
When, on the other hand, the tone is indicated by a letter in the word’s standard spelling, to omit that letter is every bit as mistaken as to misspell the word. Hence the tone cannot be omitted except by mistake. The one disadvantage of this system is that it has not been adopted as standard yet; I think Hanyu Pinyin is going to continue to be a problematic way to convey the true sound of a Chinese word until this, or a similar system, is widely implemented.
But one could start with Taipei’s streetsigns.