Sound Advice
Volume 11, Issue 2, 2008

Grade Separation Projects

Last month, the Orange County Transportation Authority issued a Request for Qualifications for the design and construction of five railroad grade separation projects. OCTA is obviously going to spend an enormous sum to construct under- or overpasses to improve transportation here in Orange County. It’s common sense that the grade separations will make the intersection of road and rail safer for motorists, pedestrians and train engineers but will they make it quieter for the local residents?

Wherever there are at-grade crossings, people who live nearby have a lot to put up with whenever a train comes through. First, there’s the irritating jangling of the crossing barrier bell accompanied by the squeal of brakes and tires as traffic pulls to a stop at the crossing. Once the traffic has stopped, there’s constant engine noise as the vehicles idle. Next, as the train approaches, comes the worst offender: the train horn. Old homes without sound insulated windows, and mobile homes in trailer parks adjacent to rail lines in older Orange County communities can be exposed to interior noise levels over 90 dB(A) from the frequent train horn blasts. That’s because Federal law demands four horn blasts (two long blasts, then one short and one long) from the train when it is one-quarter mile from an at-grade crossing.

As the train passes by, there’s the high frequency metal-on-metal screech and clickety-clack rumble from the wheel and track noise, and the low frequency throbbing of the engine. Once the train is through and the barriers lift, there’s the sound of cars accelerating across the railroad tracks, and the occasional sharp hiss of pneumatic brakes and grinding of gears as trucks get underway.

Eliminating the at-grade crossing means there’s no longer a safety issue, so trains have no need to sound their horns. No barrier alarm bells are required so that noise source goes away too. Traffic no longer needs to screech to a halt or accelerate away from the crossing; it just flows smoothly at constant speed either above or below the railroad track. 

However, there will be new acoustical impacts to consider. The fact that traffic and trains no longer have to slow down or stop at the intersection of road and rail means a consistently higher noise level because of the added speed. Especially where the grade separation introduces an overpass, the design may introduce large hard surfaces that reflect noise in new directions. This was the case in a grade separation overpass project in the city of Anaheim on which we consulted several years ago.  There, reflections from the bridge structure, the sound wall located atop the bridge, and the retaining walls for the tunnel all had to be taken into consideration in the acoustical design.

During construction of the over- or underpass, residents adjacent to the rail line will not only be exposed to train and traffic noise, but also for many months there will be the additional contribution of noise from back hoes and earthmovers and jackhammers and a whole assortment of large construction machinery vehicles. Temporary noise barriers may shield local homeowners from the brunt of the onslaught. The photograph at right shows a temporary sound wall that has been erected next to a mobile home park adjacent to current grade-separation construction at Jeffrey Road in Irvine, California.

Grade separations are complicated projects from an acoustical standpoint. They require the experience and expertise of a consultant familiar with the problems, the ramifications and the most cost-effective solutions. If you need acoustical consulting services for a project such as this, we would be glad to assist you.

« Back to Archived Sound Advice Articles