More than 120 British drivers have been the victims of sneaky thefts on the AP-7 motorway near Barcelona, according to the AA.
Thieves have been flagging cars down and then distracting their occupants while one of the offenders steals their possessions.
Richard Hibbs, from North Wales, was robbed while he was visiting family near Barcelona, on Spain's north east coast.
He was driving along the AP-7 north of the Catalan capital at 80mph when another vehicle waved him down after creating a loud noise.
After he stopped to see what the problem was, one man engaged Mr Hibbs in "animated" conversation in Spanish about his rear tyres while his accomplice sneaked out and emptied the glove box.
The tourist realised that something was amiss only when the man who was speaking to him "broke off mid-sentence, ran back to his vehicle and dived through the rear window while it was moving at speed".
"I think it's very important to have a raised level of awareness," Mr Hibbs told ITV's Good Morning Britain.
"If I had been perhaps slightly more alert on the day and thought twice about what was happening, I might not have got out of the car.
"I don't think I would stop in similar circumstances on a Spanish motorway in future."
Rosie Sanderson, of the AA's international division, said the thieves were focusing on stretches of the motorway going from airports to resorts.
She encouraged drivers planning to take to the road in Spain to "do their research" and continue to the next safest place if someone tries to flag them down.
"If you travel at a slow speed, just make sure you've got people around you before you stop," she said.
And she warned that another trick used by the thieves is to stop cars and stab a tyre before pretending to come to the rescue when it goes flat.
A Foreign and Commonwealth Office (FCO) spokesman said: "It is the beginning of the peak summer season, when many thousands of Britons will be driving foreign-registered or hired vehicles across Spain.
"Distraction techniques can be very effective with honest people who think they are being helped by a good Samaritan - when in fact these criminals are the opposite.
"We want people to be aware, wary and not be caught out."
The FCO said that occasionally some of the robberies had been violent, although most were not.
Figures for highway robberies have only been made available by police in the Catalonia region but consular assistance has also been provided to British nationals elsewhere in Spain.
Officials are working with the Spanish authorities to raise awareness of the problem and have released a video aimed at British nationals driving in Spain and elsewhere in Europe.
More than 11 million Britons visited Spain last year, according to the Office for National Statistics.