> hundreds of feet beneath you and Highway 101.<p>Not US 101 — Interstate 280. Scott Parker on AARoads sez:<p>“There is one historic oddity regarding I-280: the bridges over SLAC (Stanford Linear Accelerator Center), on Stanford University land, were constructed between 1965 and 1967, some 4 years before the remaining construction on that segment of freeway commenced -- but in conjunction with the western extension of the accelerator (a long continuous structure that extends under the freeway). This advance construction was done as a joint project between the Division of Highways and Stanford in order to ensure that any vibrations emanating from the bridge structure would not affect SLAC operations and experiments. The bridge bents on either side of the accelerator building are double-isolated, with vibration-absorbing pads between the bridge beams and the vertical bents -- and the bents themselves are sitting in a "sheath" of sand and clays to dissipate any remaining vibrations that might be transmitted to the ground; the entire bridge structure therefore "floats" above the surrounding ground rather than terminating there. It is held in place by its own weight; connected to the remainder of the freeway by a series of short metal bars over which traffic passes (driving it, one feels a series of minor "bumps" at the connecting bars as well as a bit of a "dip" in the middle of the bridge itself -- a deliberately designed "sag" due to the irregular hillside that was carved out to accommodate the accelerator structure. The unusual design of this road-to-structure isolation interface was a condition imposed by Stanford University as part of the agreement to route I-280 through the back of campus property; there are seismometers installed in the ground underneath and adjacent to the bridge to measure the continuous effectiveness of the isolation measures; when particularly precise SLAC experiments are undertaken, I-280 has on rare occasion been closed for the duration of such an experiment. This is a one-of-a-kind accommodation by Caltrans and its predecessor agency made necessary by the nature of this particular atom-smashing device.”