Use ถนนข้าวสาร if we wish to be pedantic. This would be how Thais would spell it.

^^ .........which properly makes it kow-sarn rather than kow-san 'if we wish to be pedantic'.
Personally, I love "Thinglish". I think it's great that, in order to learn a second language, one needs to learn a third. "Thinglish" consists of such words as "Ratchawongse" and "Narithiwas". Tell a taxi that you want to go to "Central" and good luck! I think Brits had a hand the "official" transliterations ...
The point is that transliteration is often a bit hopeless. Because it is Thai, because there are so many different transliterations, whatever rings one's bell should suffice.
If you want a real chuckle, try using Google translations from Thai to English. I have laughed myself to tears at times!

One of the things I like about Thailand is that there is NO "proper" spelling of Thai words when translated to English. You can write anything so long as it approximates the Thai pronunciation.

^^
Well that's the notoriously poor transliteration system adopted by successive Thai governments that conspires to make it difficult for English speakers to get even close to pronouncing Thai, without even getting into the tones! For other ridiculous examples go look at the street signs for Surawong Road and Jarerngrung Road.
As the Thai is written and said, San is clearly Sarn with a long "A" not a short one. Pronounced as written underneath most people say San, which is a short vowel and isn't how a Thai speaker says it.
Say it how you like, the backies still manage to find the ghetto!
My wife's pronunciation doesn't have a "long A" or any "R". I guess I'll have to set her straight.
It's probably due to the fact that she pronounces it based on that terrible chicken scratch Thais call their alphabet!

^^
My wife's pronunciation doesn't have a "long A"
It probably does, as short and long vowels are really important for comprehension, but you don't hear it as a long vowel since you're more used to drawled-out vowels in American English - I mean New Yorkers drink coooorrrrrrrrrrr- ffeeeeee and say that Thais pray to Boooooo-Daaaaaah.
As for the "R" well that's normal, isn't it! Only Middle Thais really pronounce Ror Reua and lots of others interchange R and L at will, but you're not a newbie, you know all this!