I assume the off-ramp was not that far from a pay phone and she walked to make the call to her parents to tell them she had run out of gas. I do not see anything about the parents having arrived to her destination to help her. I wonder how long after they got the call that they did arrive and discovered she was gone? An off-ramp on a highway is usually very busy and assuming this took place during the day, I wonder how nobody witnessed her getting into this guy's car. Would she go willingly with him even knowing her parents were on their way and after she had already turned down the highway patrol officer for help? If it wasn't willingly and he forced her, it would seem almost impossible that the scene was not witnessed by at least one passerby.
Robin placed the call home to her parents at 02.04 a.m and her father arrived at the scene around 02.30 a.m, by which time Robin had already disappeared. The CHP officers drove past her a few times to keep an eye on her and after one such trip pushed her car farther up the shoulder as it was slightly into the No. 4 lane.
All of this information was taken from a newspaper story regarding Robin's disappearance titled "Why Missing Coed Was Left on Freeway."
I've often theorized that whoever took Robin may have pretended to be a cop (on or off-duty) who promised to either take her home or perhaps to the station where it'd be warmer and safer for her.