The bomb threat called in to the Ambassador Bridge was more costly to the economy than the threat against the Detroit-Windsor Tunnel last week.

Bill Anderson, the Ontario research chair in cross-border policy, said 25 per cent of trade between Canada and the U.S. crosses the bridge on a daily basis. He called a threat against the bridge "much worse."

Anderson and Stephen Laskowski, the vice president of the Ontario Trucking Association, each said it costs $75/hr for a truck to sit idle. Monday, hundreds of trucks waited five hours for the bridge to reopen.

Detroit police said a 911 call came in around 7:20 p.m. ET Monday to authorities on the U.S. side of the Ambassador Bridge. The caller said a bomb would go off in 10 minutes along the busy freight crossing, police Inspector Don Johnson said during a news conference Monday night.

The closure came four days after the Detroit-Windsor Tunnel was shut down for nearly four hours on July 12 because of a bomb threat.

Monday’s bridge bomb threat came from the Detroit side of the border, while the threat against the tunnel originated from a payphone in Windsor, according to local police.

Detroit police spokeswoman Sgt. Eren Stephens said early Tuesday that the bridge reopened at 1 a.m. ET after security sweeps failed to turn up any incendiary devices.

"You just hope it’s not some sort of a pattern that repeats. One or two incidents won’t have a big economic impact," Anderson said. "If it becomes a continuing problem, it will have a series negative impacts on the economy."

Several manufacturers rely on just-in-time delivery of parts. Laskowski said factories can sustain a two-hour delay before production starts slowing down and eventually stopping.

"This is not good for the economy," Laskowski said.

 Anderson said the economy can sustain a six-hour delay before the lost money really begins to add up.

"They’re all part of a supply chain that stretches across the border," Anderson said of the trucks. "If those parts don’t get to the plants on time, it can shut down assembly. The cost of shutting down assembly is very large."

Laskowski said a secondary effect of the threat was the fact truckers can only be at the wheel for a limited number of hours each day, and sitting in traffic — bomb threat or not — counts against them.

"Drivers were in care and control of the equipment," Laskowski said. "Obviously we need to get to the bottom of who is calling these in.

"At the end of the day, the customer doesn’t care why it didn’t get there. The customer just knows it didn’t get there."