Despite widespread appeals, the driver, who picked up Celine Figard, 19, at a service area near Newbury, Berkshire, has not come forward.
Detective Superintendent Des Thomas said yesterday: "If he is an innocent man why has he not come forward? Either he has not heard about the appeal or you can draw your sinister implications. Someone out there knows who he is. The quicker they tell us the quicker we can get to her."
Thousands of posters showing a picture of Celine, asking "Have you seen this girl?" and stating "The police fear for her safety" are to be distributed within the next few days.
Celine was given a lift from the Chieveley service area at the junction of the M4 and A34 near Newbury last Tuesday.
The poster describes the white Mercedes cab she got into as pulling a grey trailer with a Thermo King refrigeration unit. The driver was white, 30 to 35, with short fair hair and a chin strap beard but no moustache. He was wearing a grey Top. the poster shows a videofit of the driver, compiled by a witness who saw Celine with him.
Celine, of Sey-Sur-Saone, France, had come to Britain to spend Christmas with a cousin who works at a hotel in Fordingbridge, Hampshire.