Bridge warning systems have been recommended for decades. The Key Bridge didn’t have one.

Safety experts for decades have been recommending warning systems on bridges that will shut off traffic — or at least warn motorists — if there’s a threat of a collapse, keeping cars from plunging into the water.

Baltimore’s Francis Scott Key Bridge didn’t have one.

Instead, police units were stationed on either end of the bridge early Tuesday because a construction crew happened to be working on it, enabling officers to quickly stop traffic when the container ship Dali sent a mayday call just minutes before the collapse. That wasn’t enough to save six construction workers who perished, but the death toll otherwise could have been much higher.

Now, safety experts are questioning why the bridge didn’t have even a simple system of warnings — such as something akin to the flashing guards regularly seen at railroad crossings.

“Why don’t they have systems that track ships, and if it’s clear that a ship is not in the shipping channel, it automatically puts alarms on the bridge and some actual gates to close it?” asked Roberto Leon, a professor of structural engineering and materials at Virginia Tech. “The technology to do that is out there, and they accomplished that essentially this time around through human intervention, but that’s not the way you want to do it.”

He added that “protective systems are not just a good idea, they should be mandatory.”

The potentially catastrophic danger posed by errant ships striking bridges is not a new concern, and experts have recommended restraint or warning systems to keep people from unwittingly driving their cars off of collapsed bridges for many years. A 1983 study by the National Research Council, spurred by a Florida bridge collapse that killed 35 people a few years earlier, noted that standards for “motorist warning and restraint systems” were “urgently needed.”

The U.S. Transportation Department sets minimum requirements for bridge construction, in consultation with state DOTs. Though the U.S. DOT implemented some stronger bridge design requirements after the Florida accident — such as requiring new bridges to be built with more protection for their support beams — they did not include a system that would alert motorists to stay off bridges that were under threat.

Neither the Federal Highway Administration nor the U.S. Transportation Department responded to a request for comment.

Maryland’s DOT could choose to go beyond those minimum standards and install an alerting system anyway, but it has not.

MDOT Secretary Paul J. Wiedefeld said Friday at a media staging area in sight of the collapsed bridge that the state doesn’t have any such warning systems in place but that “we have people looking into that research at this time.”

“That was 45 years ago,” he said when asked about the Florida bridge collapse. “If there’s anything immediate we can do, we will do it.”

“We will moving forward look at any and all options,” said Gov. Wes Moore when asked about a warning system. “There’s nothing I will not do to make sure the people in my state are safe.”

Incidents where a ship strikes a bridge and causes it to collapse continue to be rare, but have the potential for enormous loss of life, as well as millions, even billions, of dollars in economic impacts. There were 35 major bridge collapses worldwide due to a ship collision between 1960 and 2015, according to a 2018 research paper presented at the World Association for Waterborne Transport Infrastructure. The greatest single loss of life came in 1983 when a passenger ship collided with a railroad bridge in Russia, killing 167 after a movie theater on the ship’s top deck was sheared off by impact forces.

The Federal Highway Administration in 1980 said it was evaluating how to reduce the severity of ship-bridge collisions, which it said were common enough to “warrant consideration of the need for motorist warning systems.” The agency also noted that federal funds could be used to construct such warning systems with the agency’s sign-off.

The 1983 study noted that the frequency and severity of bridge-ship collisions appeared to be increasing. The study authors noted that “traditional margins of safety are narrowing” and that “ship-bridge collisions can easily be envisioned that are far worse than any that have occurred.”

The Port of Baltimore is a major importer of automobiles, and a major exporter of coal — all of which must now find another berth elsewhere. North of $100 million of goods comes through the port every day, according to Transportation Secretary Pete Buttigieg, in addition to $2 million in wages to the 8,000 people whose jobs are connected with the port.

The lack of significant protection around the Key Bridge’s piers has also come under scrutiny in the wake of the disaster. Leon, the engineer, said that bridge codes require engineers to consider impacts from ships, but those standards, established in the 1990s, don’t take into account the mega-container vessels that now flow in and out of America’s ports.

For older bridges, retrofitting piers to protect them can also be cost-prohibitive. Leon said that bridges should be “retro-fitted no question” but that if a requirement were to come down from the federal government, funding would need to come with it.

“This bridge was built three years before the Tampa Bay tragedy, after which we really started to look into this vulnerability,” said Rachel Sangree, associate teaching professor of civil and systems engineering at Johns Hopkins University, of the Key bridge. “It’s more difficult and costly to go back and retrofit something.”

The American Association of State Highway and Transportation Officials, an umbrella group of state DOTs, published bridge design specifications for vessel strikes in 1991, many of which were incorporated into its 1994 bridge design guide that was then mandated by the federal government. One of the provisions that got dropped: Motorist restraints.

AASHTO Executive Director Jim Tymon said in an interview that state DOTs’ chief engineers from around the country make the decisions about what specifications move forward. And motorist warnings and restraint devices are “an option that states would have to incorporate if they choose, but not one that's necessarily mandated by the federal specifications.”

The driver warning system stands out as one major recommendation in the 1983 report that wasn’t implemented. The codification of AASHTO’s bridge standards served to meet the recommendation that new bridges be designed — and old bridges be evaluated — “for the possibility of ship collisions.”

But vulnerabilities remain, especially for bridges, like the one in Baltimore, built before those recommendations were made, without a requirement for existing bridges to be retrofitted to meet them.

American Society of Civil Engineers Past President Norma Jean Mattei points to the risk analysis that engineers now have to perform for new bridges and suggested that older bridges should have new risk analyses performed from time to time as conditions change: More traffic under or over the bridge with bigger and heavier ships, and bigger and heavier trucks, not to mention environmental conditions that could speed up the ships.

“As we have climate change, possibly you'd see [water] velocities go up because of large convective storms in the catchments that ultimately go down the river,” Mattei said. “Risk changes.”

In a world of finite resources, Mattei said stopping traffic on the bridge might not be the highest priority, and transportation departments are always trying to “use the money that they have to buy down the most risk.”

“We may not want to invest in ways to stop traffic as much as we might want to invest in gathering the data and seeing when risk has changed and triggering a more rigorous analysis,” she said. “And if the risk is below the accepted risk, then you shouldn't do anything. You're good.”