！
Please refrain from hanging out the window while the vehicle is moving!
Literal
Running-in-progress [attributive-の] box-riding [topic-は] [honorific-ご]-refrain-please-[command-ください]!
Public-notice register. ~中 suffixes a noun to mark 'during / in the middle of X' — 走行中 = 'during operation, while moving.' 箱乗り ('box-riding') is a slang term for leaning or hanging out of a car window, or standing with half one's body outside the vehicle — the reckless, show-off posture associated with 暴走族 (motorcycle gangs) and yakuza films. ご遠慮ください is the polite formula 'please refrain from X,' honorific ご + 遠慮 ('refrain, hold back') + くださる command.