The sprawling and congested Saudi capital is in a race against time to complete its $22.5 billion metro system within four years, a senior official said on Wednesday.
Abdullah Allohaidan told AFP in an interview that the rail and bus development -- whose construction is changing the face of Riyadh -- is the largest project currently under way in the Middle East "and I think in the whole world."
Construction began a year ago but has accelerated in the last few weeks, with road closures, digging equipment and hard-hatted workers taking over the city's business core, to the frustration of drivers facing detours and lane-closures.
"I think the biggest challenge we are facing is the duration of the project," said Allohaidan, assistant to the metro director.
Plans call for construction to be completed by the end of 2018.
"Usually the duration for those projects is much longer," he said in front of colour-coded maps showing the metro's six lines that will cover 176 kilometres (109 miles), supported by a bus network of 1,150 kilometres.
But with the population of Riyadh projected to reach 8.2 million by 2030, up from the current 5.7 million, the city needs public transit, he said.
Saudi Arabia is OPEC's biggest oil exporter and the country's economy has been one of the best performing in the Group of 20 leading nations, according to the International Monetary Fund.
Three foreign consortiums are building the metro, with France's Alstom, Canada's Bombardier and Germany's Siemens among the major participants.