MOVEABLE BARRIER Golden Gate bridge to get moveable barrier

MOVEABLE BARRIER

Golden Gate bridge to get moveable barrier

The board of the Golden Gate Bridge authority decided after exhaustive tests and studies, and much agonizing, that it should move toward installation of a 1’ thick moveable median barrier at a cost of about $7m. The bridge has outside lanes of 11’ and 4 inner lanes of only 10’ for a total 62’ width curb to curb and presently uses soft thin tubular pilons which are moved 4 times per day for 4/2, 3/3 lane configurations to handle tidal commuter flows south in the morning and north evenings. Over 110k veh/day use the famous bridge.

It has suffered an average of 5 crossover accidents a year including some severe ones. The 8 fatalities on the bridge in the last 10 yrs were associated with crossovers. The standard 2’ wide concrete moveable barrier from Barrier Systems Inc was rejected in part because it would reduce lane widths excessively and because it was thought there might be a rash of secondary accidents.

Barrier Systems Inc, the patent holder on such systems, has now designed and tested a special steel bodied concrete-filled moveable barrier only 1’ thick (0.3m) with the GG bridge in view as its potential first customer. (TRnl#25 Mar 98 p5)

The bridge’s chief engineer Merv Giacomini in a report to the board May 1 said there was a case for proceeding, but that it could be argued either way. Giacomini’s report says it could be expected the barrier would result in the virtual elimination of crossover accidents on the bridge and that this, on balance, outweighed various uncertainties and risks of the project. An early study by Robert Seyfried of NW University which put the barrier project on hold speculated that the moveable barrier might produce an increase in secondary accidents, but Giacomini’s report notes that this has not occurred at other installations. On the Auckland NZ and Coronado CA bridges where moveable barriers have operated longest there has been a reduction in total accidents and no crossover accidents. Much the same is reported from Boston and Dallas where HOV lanes have them and from the Tappan Zee bridge of the NY State Thruway authority where a barrier is moved to create 4/3 and 3/4 lane configurations.

Giacomini expresses some concerns about the barrier’s reducing sight distance on the curving bridge approach for low cars which would have less warning of debris on the road. Existing raised lane markers will have to be replaced with recessed ones, because the barrier could damage them in moving. Giacomini’s report cautions that it cannot be certain the good results obtained elsewhere with moveable barriers will be replicated on the GG, but on balance recommends they proceed to detailed design of the system.

The oldest moveable barrier installation on the Auckland Harbor bridge, New Zealand has a sharp approach curve and an even narrower roadway — 42’ curb-to-curb in the central 4 lanes section. They had an average 3 crossover fatalities/yr before the barrier was installed in 1990 and have had no crossover accidents since it was installed. An average of 1.3 times/yr the barrier has been hit and moved a distance into the opposing lane, but no secondary accidents have resulted.

A report (not known to Barrier Systems Inc, but which we got from NZ) on the operations of the Auckland barrier prepared by a NZ consultant for Seyfried of NW University who is advising the GG bridge, says the barrier there has “proven to be a very cost effective road safety project. It has eliminated the trauma and expense of cross-median fatalities while safely enabling smooth flow of commuter traffic by tidal flow means. (It) continues to operate every weekday without failure and is continuing to save lives.”

Barriers have another benefit. A certain proportion of motorists will not drive in the center lanes of non-barriered multi-lane roadways for fear of cars in the opposing direction straying into their path. On the GG bridge they are known as “suicide lanes” according to the SF CHRONICLE. As a result these center lanes have a lower carrying capacity in most unbarriered facilities. Installation of a barrier reduces that fear of a head-on and thus the capacity of the central lanes is enhanced.

Truck flipovers

Of course there is no perfection. Between heights of 32” (81cm) and 54” (137cm) there are a series of trade-offs in fixed barriers, engineers note. The greater height provides protection against higher vehicles at larger angles, and cuts headlight glare. But the downside is that it can severely reduce sight distance on curves and give motorists driving close a sense of being too walled in.

The lower height barriers will in certain hits by high vehicles flip them over. Just May 23 on I-95 in Chester PA a gasoline tanker truck heading into Philadelphia swerved to avoid rearending a car, lost control, hit the fixed median barrier, flipped over the barrier and landed upside down in the path of opposing traffic. The load of fuel burned, killing the tanker driver and also a driver in a pickup traveling southbound. The heat of the fire buckled steel bridge beams underneath the pavement putting half the roadway out of action, and calling for reconstruction from the abutments up. From photos the barrier looks to be a low one (32”) even though here the road is straight and it could have been built higher to reduce the chance of a flipover.

Fixed barriers have no give. The moveable barriers are designed with some give, being laid flat atop the pavement and linked to their neighbors by steel pins to form a chain, which is stretchable. The amount of give can be controlled by the tension applied to the chain of barrier blocks by the transfer vehicle that lays them down. This designed ‘give’ in a moveable barrier may reduce the danger of flipovers, but an official of Barrier Systems says this has not been tested experimentally with large trucks. The system has been around a bit over 10 years and with worksite systems there are about 50 in service, a length of some 200km (120mi). There have been a variety of hits by trucks, displacement of the barrier, and damage to trucks and barrier, but so far no case of a flipover. (Contacts: Merv Giacomini GG Bridge 415 923 2250, Warwick Mason Transit NZ 9 377 7092, Bob Seyfried NWU 708 491 5040, Barrier Systems 702 885 2500)