"The most complicated problem was getting there, to locations totally inaccessible, and bringing huge quantities of materials," said engineer Jose Refugio Avila Muro, a federal subdirector of highway projects for Sinaloa state. He compared the topography to an electrocardiogram: "Lots of peaks, and you have to find a way to get to each peak from below. You just keep going, one by one, to each new point of construction."
The new highway will cut the drive between Durango and Mazatlan to 2.5 hours from the current six hours of hairpin turns, few guard rails and the Devil's Backbone, a stretch of road along the spine of a mountain with drops of hundreds of meters (feet) on either side.
Coming around a blind curve, a driver may suddenly have to negotiate passage between a semitrailer barreling downhill and a handful of cows tiptoeing along a narrow shoulder. Deadly accidents are common. A bus carrying mostly retiree tourists to Mazatlan plunged off the road a year ago, killing a dozen and injuring 22.
But the old highway is not the most forbidding part of the landscape.
From December 2006 until September 2011, when the federal government stopped providing numbers, Sinaloa and Durango on either side of the Baluarte Bridge were among the deadliest states in terms of drug-related killings. Mazatlan ranked 8th among Mexico's more than 2,400 municipalities and Pueblo Nuevo, the municipality on the Durango side of the highway, was 35th most violent up to the end of 2010.
The U.S. State Department discourages travel in both states, except for specific tourist zones of Mazatlan.
The killings spiked in townships near the new road as a group known as the Mazatlecos and the Zetas battle for territory controlled by the Sinaloa Cartel, named for its home state and headed by Joaquin "El Chapo" Guzman, the world's richest and most-wanted drug lord. A series of attacks around Concordia, Sinaloa, in late 2012, including the Christmas Eve massacre, caused some 250 families to flee their communities, said Concordia Mayor Eligio Medina. They have yet to move back.
Medina said the new highway could change the criminal dynamic, bringing tourism to colonial Concordia, founded in 1565 by the Spaniards as a way station between the coast and the gold mines. It's also one of the most biologically diverse townships in the world, he said, noting that a new species of plant, the ageratina concordiana, was recently discovered there. He envisions everything from bird-watching to bungee jumping in Concordia's Chara Pinta ecological preserve.
"The road will increase jobs and keep people busy," Medina said. "When there is social mobility, criminal groups are more limited."
Medina said the area is quiet again, with the Mexican military patrolling the towns that were attacked. Mazatlan tourism officials say killings there have dropped from 307 in 2011 to 43 so far this year. Latin America security expert Samuel Logan agrees the new road could be a boost to tourism and commerce, and but also to illegal transport.
"Maybe Concordia will grow and there will be a Holiday Inn Express there," he said. "Will there be running daytime shootouts on this highway? Not likely. But will there be convoys of eight to 10 trucks going 90 mph (140 kph) filled with guys with guns? Probably."
Associated Press writers Martin Duran in Culiacan and Karla Tinoco in Durango contributed to this report. Follow Katherine Corcoran on Twitter, @kathycorcoran