The southbound span of the Lake Pontchartrain Causeway — the first of the two spans built — was an engineering marvel when it opened in the late 1950s. Nearly 60 years later, though, one aspect of the bridge is proving inadequate for the modern age: the concrete railings, which are bordered by a 10-inch curb and rise only 25 inches above the bridge deck.

While they may have been fine for the low-profile cars of the 1950s, the railings are often unable to halt today’s bigger vehicles, especially pickups and SUVs. Atop the concrete railings sits a handrail, which may look sturdy but actually is intended only to assist people walking along the curb. It does nothing to stop a car from going over the side into the water 25 feet below.

On Wednesday, a Texas A&M University engineer recommended that the Greater New Orleans Expressway Commission install a steel double rail atop the concrete barrier on the southbound bridge, saying it would significantly improve the bridge’s safety.

William Williams, of the Texas A&M Transportation Institute, said crash tests performed by the institute showed that a double rail would keep vehicles as big as a large moving truck from going over the side of the bridge.

The double-rail system was one of two that Williams said would work. The other option, a single rail, also passed the crash tests but didn’t perform as well as the two rails.

Williams was part of a team hired by the commission to find a way to retrofit the causeway’s railings to make them safer.

The numbers aren’t good. Since 1994, there have been 14 “overboard” accidents on the causeway, those in which a vehicle went over the side, and 13 of those have been on the southbound side. Eleven of the 14 resulted in fatalities. And the rate seems to be increasing: There were a total of six such overboards in 2013 and 2014, all on the southbound side. Four of those resulted in fatalities.

Much of the blame has focused on the low railings on the southbound span. Last year, when Andy Pate lost control of his truck and hit the side of the southbound bridge, he said the curb and railing had an effect akin to a “ ‘Dukes of Hazzard’ ramp,” sending his truck over the side.

Thus, Williams’ study focused on a way to keep cars and trucks on the bridge. The group tested three vehicles — a compact sedan, a pickup and a large moving truck — against four different rail designs.

The double rail would raise the height of the 25-inch southbound span rail to 46 inches, Williams told the commission Wednesday. The single-rail option would raise the northbound span’s rail to 45 inches and would be a good match on that side, he said.

With those recommendations in hand, causeway officials can now turn to the true challenge of improving the bridge’s safety: finding the money.

The commission has been unsuccessful so far in applying for federal grants to help pay what could become a nine-figure price tag.

To help estimate the costs, the commission’s consulting engineering firm, GEC, has asked several contractors to provide a rough estimate of what installing nearly 100 miles of rail — rails on each side of both spans of the nearly 24-mile bridge — could cost.

One significant challenge is that the rails will have to be installed while the bridge remains open, so companies have been asked to make their estimates on the basis of various criteria, such as how many hours a day crews could work and how much of the work could be done by barge, according to Cary Bourgeois, of GEC.

The commission hopes to have that information in hand by September or October, at which time a serious search for funds could commence, according to Stephen Romig, the commission’s chairman. One option may be federal transportation infrastructure loans, which offer low interest rates, he said.

One other option that has been suggested in the past — raising tolls on the bridge — remains on the table, Romig added. Tolls have been raised only once in the bridge’s history, from $2 to $3 for a cash round trip. With a toll tag, the trip still costs just $2.

At a public meeting in Mandeville last year, opinions trended generally in favor of raising the tolls if it improved the safety of the bridge.

Follow Faimon A. Roberts III on Twitter, @faimon.