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Compared to the personal occupant vehicle (i.e., cars, SUVs and pickup trucks) world, an almost negligible number of public transportation accidents involve vehicle-to-vehicle collisions. In marketing the safety of their services, some modes (the schoolbus community in particular) boast that far more incidents occur outside the bus than inside it — although this admission hardly comprises an accomplishment in my eyes. Otherwise, the sense one might ordinarily have of this anomaly is distorted by the fact that vehicle-to-vehicle collisions are far more often publicized than most other public transportation-related incidents or accidents — and most incident scenarios are rarely if ever reported (presumably because someone falling down a stepwell, e.g., does not sound particularly interesting or is not "newsworthy").
This reality is all the more striking given the tight schedules, fatigue, and marginal monitoring and refresher training and other compromises that exist on the driver's side of the public transportation equation. Yet this equation would seem more heavily-weighted in favor of vehicle-to-vehicle collisions on the vehicle side of it: Not only are public transportation vehicles significantly heavier in mass than most fellow vehicles they might collide with (which translates into longer stopping distance), but particularly with large vehicles containing air brakes, this feature adds roughly an additional half-second to a driver's "reaction time," as the air in the system must be compressed before it can depress the brake cylinders (unlike the liquids used in the hydraulic brake systems of lighter vehicles).
One factor that helps to explain this phenomenon is the commonality of defensive driving typical to many modes' training programs. Because certification for defensive driving is a requirement to obtain a commercial driver's license, this means that the driver of any vehicle weighing more than 26,000 lbs. GVWR has been certified for such training, as is the driver of a schoolbus of any size and mass. Thus, all schoolbus, transit and motorcoach drivers are certified for having taken this training, and it is a common element in most complementary paratransit services – since the "lead agency" is almost always a transit agency (whose fixed route drivers receive this type of training). Also, the provision of training in defensive driving is almost always cost-free: Most highway patrol departments, and the National Safety Council, provide it for free. (Of course, transportation providers must usually pay their drivers to attend the training.)
These observations lead one to suspect that when vehicle-to-vehicle collisions do indeed occur, a broad spectrum of other errors and omissions have likely led to it beyond a driver simply following too closely, driving too fast for conditions, or not paying attention. While driver fatigue is the most common culprit, this phenomenon is rarely examined in vehicle-to-vehicle collisions unless they are catastrophic — and in such collisions, driver fatigue (along with negligent driver assignment) is typically a principal cause of them.