High oil prices have forced a reduction in the national speed limits in Spain, with main roads now restricted to 110km/h (68mph).
Workers toiled throughout the night to alter the 120km/h (75mph) signs, with 6,000 now bearing 110km/h stickers.
The slowing of the national limits is intended to limit the amount of fuel used by cars and lorries in response to rocketing oil prices caused by the unrest in Libya and elsewhere in the Arab world.
Spain is said to be heavily dependent on imported oil, and 13 percent of its fuel comes from Libya.
The Spanish government says it is a temporary measure intended to last just four months, and it hopes it will account for a 15 percent reduction in the nation's fuel usage over that time.
"We are going to go a bit slower and in exchange we will consume less petrol and pay less money," said Spain's deputy prime minister Alfredo Perez Rubalcaba.
But the minister claims the move is not down to a lack of fuel, and it is more for financial reasons.
Ismael Sanz, economist at King Juan Carlos University in Madrid told the BBC: "If people are devoting a higher share of their income to imported fuel, then they're spending less buying Spanish products, going for tapas."
Even more sceptical observers have suggested that the government is intending to make up the shortfall in taxes on fuel through an increased amount of speeding fines from drivers not noticing or simply ignoring the new speed limits.