Wednesday, April 30, 2014

How fast was the Berkeley County bridge daredevil driving?

We asked some physicists to run the numbers

Posted by Paul Bowers on Wed, Apr 30, 2014 at 4:55 PM

click to enlarge Here's what the bridge looked like before it collapsed. - GOOGLE MAPS
  • Google Maps
  • Here's what the bridge looked like before it collapsed.

As you may have already heard, a bridge collapsed Monday night in Berkeley County, right as a pickup truck was speeding toward it. Rather than stop, the driver kept driving and somehow managed to clear the 15- to 20-foot gap that had suddenly formed in the roadway.

The driver, Jason Goodyear of Moncks Corner, has become a minor folk hero, with some comparing his truck jump to a classic Dukes of Hazzard stunt. He isn't giving interviews, but we have one burning question: Just how fast would a fella have to drive to clear that gap?

The question sounded like a textbook physics problem, so we put it to some physicists at the College of Charleston and got the following answer:

52 mph.

That figure comes from Physics Department professors Linda R. Jones and Jeff Wragg, and it was based on an early assumption that the "launching ramp" side of the bridge was more or less level with the "landing" side of the bridge (see a note from the two professors below explaining their methodology). If you factor in later reports that seem to show that the landing surface was actually about two feet below the launching surface, Wragg says the required speed would have only been about 29 mph.

In other words, whichever way you slice it, Goodyear didn't even have to break the speed limit to clear that jump. The speed limit on Cypress Gardens Road, the site of the bridge collapse, is 55 mph.

Now, we can't say we recommend trying a bridge jump like this for yourself. Goodyear's truck sustained serious damage when it landed, and Berkeley County Rescue Squad Chief Bill Salisbury told the Post and Courier that he'd gladly go half in on a lottery ticket with Goodyear because he's a lucky man. But in case you're still curious, here's the data we provided to Jones and Wragg:

Goodyear was driving west in a Chevrolet Z71 4x4 on Cypress Gardens Road when he drove across a gap in a bridge. According to the SCDOT, the road has a 5 percent incline headed west, so the road formed a slight ramp with a launching angle of about 2.86 degrees. The size of the gap has been reported as between 15 and 20 feet, so we'll split the difference and call it 17.5 feet (or 5.4 meters).

Here's the formula that Jones and Wragg used, where g is standard acceleration due to Earth's gravity:

Flying Truck 1.1


Again, it's worth noting that the preceding formula assumes that the launching and landing surfaces were level with each other. To incorporate a two-foot drop, Wragg used the following method to determine a minimum launch speed of 29 mph:

Flying Truck 2

This story originally included a picture of the wrong railroad crossing. We regret the error.

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