Texas DPS said they responded to the crash at about 8:06 a.m. on the Grand Parkway westbound at Birnham Woods.
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Officials told ABC13 that the box truck went off the road during the crash, landing on the lanes below the overpass, while the 18-wheeler's cab ended up hanging partially over the overpass.
Amazingly, both drivers survived, with the 18-wheeler driver being pulled out. The box truck driver was taken to the hospital with critical injuries.
Photos from the Spring Fire Department posted to Twitter just before 9 a.m. showed the perilous aftermath.
South Montgomery County Fire Department is the agency that rescued the driver from the dangling truck. Battalion Chief Nathan Huffman said they secured the 18-wheeler and then used a ladder designed for high-rise rescues to get the driver safely out.
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"He was very appreciative," Huffman recalled. "He was very thankful. He did have a lot of kind words to say to our members, and it was nice to be able to rescue him, and he was unharmed."
Authorities said the crash happened at a time when there is usually heavy traffic in the area.
"We were (really) lucky. You know you got three schools right behind you, and we got debris on the ground, the driver on the ground," Montgomery County Commissioner James Noack said.
The Grand Parkway westbound was closed at Birnham Woods for several hours while crews work to clean up the wreckage. It opened to a single lane a couple hours after the crash.
TXDOT told ABC13 it inspected the bridge, and despite the weight of the dangling truck, it's structurally safe.
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Exactly how the crash happened is under investigation.
Watch: ABC13 coverage of 18-wheeler collision on Grand Parkway in Montgomery County
Fire crews rescue driver from big rig dangling off Grand Parkway
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