Curious Iowa: How do cable barriers stop cars?

Iowa has been using cable barriers on its highways since 2003, as an alternative to solid concrete barriers, to stop drivers who leave the road from crashing into cars traveling in the opposite direction.

Cable barriers are made by pulling multiple woven steel cables taut between evenly spaced steel posts. The cable barriers in Iowa are between 28 inches and 39 inches tall.

But can a metal cable, installed a few feet off the ground, really stop a speeding car? Or a semi? That was the question from a reader of The Gazette, who wrote to Curious Iowa to ask the question. Curious Iowa is a series from The Gazette that seeks to answer Iowans’ questions about the state, its culture and the people who live here.

We talked with Iowa State Traffic Engineer Chris Poole about the science behind the cable barriers and how effective they’ve been in stopping crashes since they were first introduced to the state 20 years ago.

How do cable barriers work?

Cable barriers are different from other barrier options, like concrete blockades, because they don’t bring vehicles to an immediate stop through brute force. Instead, the cables latch onto the car — between the tires and the frame of the car or on the side mirrors — and move backward slightly, bringing the vehicle to a stop more slowly, according to Poole.

Before any cable barriers were installed in Iowa, they went through rigorous crash testing with small vehicles and larger pickup trucks. The barriers were not tested against semi-trucks before being installed, but Poole said they’ve proven to be effective in stopping most semi-trucks that have run into them.

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“The posts hold the cables up, the cables deflect backward, grab onto the vehicle, slow the vehicle down, and bring it to a gentle stop. Because there’s that movement, it’s not such a harsh, all-of-a-sudden crash that occupants experience. It’s a much gentler experience,” Poole said.

The posts that hold up the cables are usually pulled out of place when a crash occurs, but Poole said they’re designed to be easy and cheap to reinstall.

The posts are placed 10 to 20 feet apart, and each one sits in a concrete foundation with a metal sleeve in it. So, fixing a barrier after a crash is as simple as pulling the broken post out of the sleeve, sliding a new post in, and rehanging the wires. The state has two on-call contractors — Dave Gryp Construction of Williamsburg and Nationwide Construction Group out of Richmond, Mich. — that fix the barriers when needed.

“Another nice thing is, because those cables are kept under tension, even when you start taking away some of the posts, or the posts are damaged, usually those cables should still maintain close to their original height,” Poole said. “So, in theory, if another vehicle were to come along before we could actually repair it, if those cables are still at the proper heights, they could capture that second vehicle as well. I don't know that that's ever happened, but theoretically it could.”

Why aren’t cable barriers installed in the middle of the median?

Poole said another common question he gets about the barriers is why they aren’t installed directly in the middle of the median, but are usually closer to the sides of the road. He said there are a few reasons for this, but it mostly comes down to the results of the initial crash testing done on the barriers.

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The standard practice across the state is to install the barriers 12 feet from the nearest lane of traffic. Poole said crash testing showed the barriers were most effective at this distance.

Having the barriers closer to the road, rather than in the middle of the median, prevents cars from getting too far onto the slope that usually exists between the two directions of traffic. When a car gets onto the slope it tends to bounce more and can even start rolling, which can make it harder for the cable barriers to latch on and stop the car, according to Poole.

“The other problem with installing down the middle of the median is that's where all the water goes, that's where all the snow goes. We really rely on a nice solid foundation to make sure that those posts can stand up straight and the barrier still has the strength needed to capture a vehicle. We don't want to put it in places where the soil is constantly wet, and the strength of that soil then would be reduced,” Poole said.

How effective have cable barriers been at preventing crashes?

Iowa has now installed cable barriers on 454 miles of the state’s 782 miles of interstates, and data show that as more barriers are installed, fewer people are killed in cross-median crashes.

In 2006, the state started tracking the number of fatalities each year from cross-median, multivehicle crashes on interstates. The highest number of these fatalities happened in 2008, when there were 16 deaths. The number has dropped consistently since then, with significant decreases in 2011 and 2012. Most of Iowa’s current miles of barriers were installed in 2011.

“When you’ve got one vehicle going 70 miles an hour, it crosses the median and hits another vehicle going the opposite direction at 70 miles per hour, the results of that almost always are catastrophic,” Poole said.

Poole said when the state first started using the cable barriers, people were concerned that they wouldn’t work, and would often complain about barriers going up in their area. Now, Poole said the complaints have decreased and it’s much more common for community members to reach out and ask for the barriers to be installed on certain stretches of road.

The state commissioned Iowa State University to perform a study in 2015 to determine whether it was worth the cost to continue using the barriers. The study determined the cable barriers are an effective use of state funds, and created a formula for determining where the barriers were most needed across the state. Poole said the state has now installed cable barriers in almost all of the locations identified in the study as having the highest need.

“I think the data has really spoken for itself and showed that they're very effective at reducing those really serious cross median crashes,” Poole said.

Have a question for Curious Iowa?

Curious Iowa is a series from The Gazette that seeks to answer readers’ questions about the state, its culture and the people who live here. Tell us what you’d like us to investigate next.

Comments: (319) 398-8328; emily.andersen@thegazette.com