On the Gibraltar side, a rosy-cheeked bobby with a West Country burr directs you politely to the interminable queue of cars waiting to get out.
Many in the queue are Spaniards on their regular cheap-petrol run, or Gibraltarians eager to check on their properties along the costa. Pedestrians rarely wait much except at rush hour, when some 2,000 Spaniards employed on the Rock queue to get home for dinner.
The tension, when it flares, comes from Madrid and London. Here on the border, Gibraltarians and Spaniards get on fine. For centuries they have been trading, smuggling, marrying each other and jumbling up their languages. Mostly, they like each other.
Jose Gomez, from the scruffy Spanish border town of La Linea, has been crossing every day for 15 years to work in a hotel bar on the Rock.
Francis, who runs a pharmacy in Gibraltar's Main Street, is married to Kathy, an elegant Spanish woman from La Linea. They cross to Spain several times a week so the children can see their grandparents. On this occasion he is braving the queue to deal with some paperwork for a flat he has bought "across the road".
The Spanish government recently called Gibraltar's colonial status a ridiculous anachronism, and described it as an "economic parasite". But Jose, and thousands like him, strongly disagrees.
"Gibraltar is like a factory for us. There is no bigger employer for miles around. No one in La Linea wants Gibraltar to become Spanish," he insists.
Jose was among some 2,000 La Linea residents who noisily protested against Madrid at the weekend for tightening screws on Gibraltar. The workers are angry that their jobs on the Rock are threatened, fearing greater hardship in this impoverished area where unemployment is 40 per cent, the highest rate in Spain.
La Linea's conservative mayor, Jose Antonio Fernandez Pons, last week urged Madrid to aid his "totally stagnant" town. "La Linea and Gibraltar are linked by blood and geography," says Mr Pons. "We need mutual understanding on a day-to-day basis. The prosperity of the area depends on it. We've always been ignored by the central government in its policy towards Gibraltar. Now we want compensation from Madrid for the hardship we're suffering."
La Linea was born because of the British colony. For centuries the people of the town trudged across the causeway to service the imperial garrison on the Rock. When Franco closed the border in 1967 he choked off the city's lifeline. "He built a factory along the bay that closed within weeks," Jose recalls bitterly, "and a football pitch in full view of the Rock, to make Gibraltarians think we were prospering."
Francis, his week-old BMW gleaming in the queue, remembers how his father struggled to keep the pharmacy going during the 18-year blockade that followed. He believes Gibraltar could survive as a banking and business centre if Spain opened up and Britain "were generous with opportunities". But few are so confident.
Gibraltar's anxiety is that London will cast it aside now that its historic usefulness is gone and good relations with Spain become the more pressing need. Fearing that Britain's commitment will wane, the Gibraltarians plead for reassurance that they won't be abandoned, sensitive to any equivocal silence.
Anachronistic they may be, but Gibraltarians say it is not their fault that Britain's 300-year colonial rule made them what they are: neither Spanish nor British, but Mediterranean Latins steeped in generations of British customs, education and habits of government. At home in neither country, they cling to the identity they have.
Across the road, back at the bar that faces British sovereign territory, Paco breaks me off a sprig from a bunch of olive leaves in a tumbler on the counter. Is this a peace offering? I joke, in Spanish. He's non-committal and replies, in English: "One more coffee? Before the bus comes."