The official abbreviation is HWY (according to the US Postal Service). You can find a full list of abbreviations like that at the Postal Service's website.
Would you please tell me, is there any abbreviation for "highway"? Here in Russia we have houses standing on highways and when the letter is posted how to write "highway" on the envelop? There must be some abbreviation as it is for street - st, for example? thank you in advance.
Thank you very much for your answer and for the URL. It will be very useful in my work! Best regards!