Inertial and Centrifugal Forces and Suspension Systems
While traveling in a forward or rearward direction, the longitudinal movement of any floor surface exerts inertial forces on the passengers and other objects resting upon or attached to it (the reason seat anchorages of each seat leg on a bus less than 22 feet in length must withstand 3500 psi of force), and the lateral movement of any floor surface exerts centrifugal forces on the passengers or objects during turning, cornering, merging, weaving, pull-ins and pull-outs. The vehicle's suspension system — a spectrum of elements encompassing axles, air bags or springs, shock absorbers, trailing arms, wheel bearings, axles, wheels and tires, among others — is designed to counteract these forces by keeping the floor level, or slightly tipping it, where an unattached object (i.e., a passenger) would otherwise be thrown forward during deceleration or braking, rearward during acceleration, to the right from a left turn, and to the left from a right turn. And to a lesser degree, this system "dampens" the vertical forces from dips (e.g., potholes), bumps (e.g., speed bumps or speed humps) and other roadway aberrations.
For the most part, transit buses and motorcoaches contain pneumatic suspension systems (air rushes in and out of "air bags" resting alongside each wheel to counteract these forces), which provide a safer and more comfortable ride than the truck-industry spring suspension systems typically employed on school buses, a range of minibus conversions, and increasingly, full-size buses and coaches built on various truck chassis. During the past two decades, a range of intermediate "hybrid" suspension systems (Mor-Ryde, EZ Ride, e.g.) installed at the OEM ("original equipment manufacturer") level, or retrofitted by a vehicle "converter," have become available, and which improve safety and comfort to various degrees. More recently, another new company began producing complete air suspension systems that, with surrounding bracketry, can be installed into a vehicle conversion in place of the spring suspension system of the van or minibus chassis on which the conversion was based. Regardless, the absence of pneumatic suspension systems is the reason passengers should not ride as standees on such vehicles (whereas they should not ride as standees on motorcoaches equipped with pneumatic suspension systems): Because their interiors are not outfitted with horizontal and vertical stanchions or grab handles, standees can easily be knocked off their feet, much less fly from one end of the vehicle to another (or crash into a window) by even the moderate application of inertial or centrifugal forces – sometimes simply from the vehicle making an abrupt stop (which would normally not even create a single g-force).
At the operating level, many management personnel are completely ignorant of these factors. Similarly, drivers of virtually any and every mode are rarely taught about inertial and centrifugal forces and their impact on unsecured (and even seated) passengers. This ignorance and lack of training and awareness — often combined with schedules that are too tight — leads to the plethora of wheelchair tipovers, and incidents involving standees (e.g., when the vehicle stops short when cut off, or to otherwise avoid an even worse incident). Yet while the importance of these forces is paramount, and links the driver's performance to his or her vehicle's characteristics, it is surprising how little attention is often paid to these phenomena by not only public transportation providers, but attorneys representing passengers killed or injured as a result of a spectrum of errors and omissions related to them.