THE RECORD:Moveable barriers

The newest moveable barrier is also the longest so far — 15km. Currently being installed on the H-1 mwy between Honolulu and Pearl Harbor it will provide contraflow HOV lanes for traffic to and from the famous naval base area just west of the city. The Golden Gate bridge is evaluating the system, and that toll authority has stimulated two innovations from the patented supplier Barrier Systems Inc:

• a new slimmer steel-clad model — the HardBody Barrier?

• an optical tracking system in place of the electromagnetic tracker for the mover

Marketing Manager Dave Crespo tells us they are almost finished testing of the optical tracking system and hope to have a proven robust setup within months. Moveable barriers so far installed rely on a precisely placed electrically charged wire buried in a sawcut an inch or so deep in the pavement. A sensor in the transfer truck follows the wire and its steering is controlled to lay the barrier precisely and smoothly in its new alignment. But like loops for traffic signals, the in-pavement wire is a high maintenance item.

Moveable barriers (MB) have been around about 10 years. Meter (39.4”) long 600kg concrete barrier sections linked by steel hinge pins are lifted a few inches off the ground by a specialized straddle truck that runs along at 5 to 8km/hr taking the chain-like barrier in one side on diagonally inclined rubber wheels which grab the units by the neck, move them underneath and re-laying them anywhere from 1.2m to 5m (4’ to 18’) to the side. The trucks have steering at back as well as front, and can drive in a ‘crabbed” fashion for maximum transfer distance.

The Pennsylvania Turnpike was an early pioneer buying 10km of MB in 1988. They were used to open a lane otherwise used for construction for rush hour trafffic each day in the Philadelphia area. It’s now for sale. An engineer there told us “It’s an excellent product but we don’t have any immediate use for it now and if we use it again, we’d expect a contractor to provide it as part of a contract bid.”

The Penna pike is doing major reconstruction in which they proceed through a 5-stage traffic movement and they use common 12’ (3.65m) either 48” (1.22m) or 52” high (1.32m) precast Jersey shape units which usually stay in place months protecting the workzone. Weighing about 2.5t and held together by a steel key, the solid Jerseys don’t need any special equipment to move, just a sling on a loader or a large forklift. These shapes cost about $300 each so a kilometer costs around $80,000. They are becoming a familiar feature at work zones around the country.

The Quickchange moveable barrier system can allow the contractor to be given an extra lane to work in except when traffic gets too heavy. Then the transfer vehicle simply runs along and gives the lane back to the traffic. It comes in two grades, a workzone grade costs around $300k/km. The fancier permanent moveable barrier is about twice that price, about $600k/km. A lot more than the big Jerseys of course, but often much less than the construction of an extra lane, even when you estimate the present value of future operational costs, given that most extra lane costs $2m to $3m/km. It is especially useful in tidal flow situations on urban motorways where one or two lanes can be made to do double duty, by using the moveable barrier to safely reverse the traffic direction mornings and evenings.

On the Coronado CA, Tappan Zee NY, Roodevelt VA/DC and Auckland New Zealand Harbor bridges the deck is continuous and the moveable barrier represents the only protection against head-on collisions. The inner lanes of unbarriered bridges have a lower throughput than average lanes because a proportion of drivers simply won’t use them — they are uncomfortable driving unprotected close by traffic in the opposing direction. A barrier will therefore increase the capacity of inner lanes.

On I-30 (Thorton Fwy) east of Dallas 14km of MB has been in use since Sept 1991 on a 2x4-lane mwy. Barrier on each roadway allows them to run 5/3 in the am-peak, 3/5 in the pm-peak. A similar setup happens on the I-93 (Southeast Exwy) in Boston, the A-13 in Montreal and the PR-22 in San Juan PR. We talked to the TX and MA operators and they like the systems. Only problem they mention is that when it starts motorists rubberneck, so there’s some traffic turbulence. The TX system had some startup problems but they got worked out, though DART had BSI take over the operation of the system. There have been plenty of incidents with vehicles hitting them, but no crossovers. They reduce accident severity and the incidence of violations compared to buffers and cones.

The I-15 proposal is the most radical suggestion for use of MB. (see nearby piece) But on I-680 in the Bay area there’s a tidal flow south in the morning from the east Bay in to jobs in Silicon Valley. BSI has suggested a staged approach where the barrier could create a quick contraflow lane taking an existing non-peak roadway lane am and pm, then be used to protect construction of an extra lane each side, finally used to support a short-hop concurrent flow HOV on one roadway and a long-hop contraflow HOV on the other. They could be used to separate toll lanes from free. The possibilities are many. (Contact Dave Crespo BSI 702 885 2500)