A tractor trailer exploded into a fireball after hitting the Route 29 bridge over the Northway on Tuesday evening, State Police said. The 5:55 p.m. crash shut down the northbound lanes just south of Exit 15 and stalled traffic for hours, Sgt. John Gaba said. Gaba said the driver escaped the flames, but it is unclear if the driver escaped injuries caused by the crash. “The crash is still under investigation,” Gaba said. “Before he hit the bridge shoulder, we got calls that the driver was driving off to the shoulder of the road and flinging up dirt. Before we could stop him, he hit the bridge.” Gaba suspects the driver could have fallen asleep or had a medical emergency. No other vehicles were involved. State Department of Transportation crews were at the scene, inspecting the bridge for damage, and the lanes were still closed at 10 p.m. Shortly after the truck explosion, the right shoulder of the Exit 14 ramp was blocked for a while by another crash. “It’s a huge mess,” Gaba said.