Conrad Hilton smashed his daddy's Range Rover head-on into another woman -- just a few hours after he dodged jail time for another high-speed wreck ... according to a new lawsuit.
Kelly Auld says Paris Hilton's troubled little bro was driving erratically in the white Range and drifting into her lane. In the docs, she says he accelerated, and clipped another car before smashing into hers.
Auld says in the seconds before the crash, she saw Hilton looking dazed with BOTH hands above his head as he careened toward her. In the suit, she says witnesses saw Conrad throw a bong out his window, and he was covered in vomit when he got out of the SUV.
The vehicle was registered to Hilton & Hyland, Rick Hilton's real estate firm ... according to the suit.
She's done her research too -- Auld points out Conrad entered a court-ordered drug treatment program last year, and had been prohibited from driving.
And get this ... she says the accident happened May 19 -- the day after Conrad struck a plea deal for a 100 mph wreck near Palm Springs.