Turning Accidents

Not for no reason, many turning accidents involve vehicles with long wheelbases—most commonly schoolbuses (because there are far more of them) and transit buses (because their schedules are almost always too tight). Even with automobiles, more collisions occur at intersections than elsewhere else. But the challenges are much greater with vehicles with long wheelbases, since the path taken by the rear tires—in a proper "square" turn—is not the same as that taken by the front tires. A further complication is that many intersections and their elements (placement of limit lines, sharp right angles of corners, etc.) are not well designed to accommodate turns made by these vehicles.

Some Basics: The Mechanics of Turning and the Anatomy of Turning Accidents

                A vehicle with a long wheelbase should turn dramatically differently than one with a short wheelbase, especially where the street into which it is turning is relatively narrow:

  1. The vehicle should pull forward into the intersection until the rear axle lines up with the "extended curb-line" of the near-side of the intersection.
  2. The vehicle should then come to a complete stop (actually required in some states or by some transit districts), or at least slow to a crawl.
  3. The driver should then "swing" the nose of the bus as sharply as possible to the right or left and begin moving forward.
  4. As this occurs, the vehicle pivots on the left rear tires (for a left turn) or right rear tires (for a right turn), and as the nose of the bus moves forward, it drags the rear tires toward it on a diagonal—like the hypotenuse of a right triangle.
  5. A bus-length or truck-length or so after the turn, the tail of the vehicle catches up with its nose, and by then, both are generally on the appropriate side of the roadway.

                From this description, it is easy to see why the failure to make a "square" turn often results in accidents. Part of this stems from the need for the vehicle to come to a complete stop, or close to it, with a proper turn. This gives the driver a chance to look around. So if the driver begins the turn only to find a pedestrian in the vehicle's path, the vehicle will not be traveling very fast, and will not have traveled very far during the driver's reaction time, and its braking distance (increasing exponentially with increases in speed) will be short.

                In contrast, if a large vehicle, instead, makes a "rolling turn," the nose of the bus will often turn into the oncoming lane of the perpendicular roadway—where it might smash into a vehicle waiting at the limit line of this street if that line is placed close to the intersection. Or it may run over a pedestrian in a crosswalk. Unfortunately, one can take this left-turn shortcut at a much higher speed than that needed to make a proper, "square" turn, since it can make a left turn at a much higher speed than a right turn. (Again, because the "arc" of a left turn is much broader than that of a right turn.)

                Unless there are at least three perpendicular right lanes to turn into, or the corner is rounded, a long vehicle will spend some time during a right turn in the oncoming perpendicular lane or lanes. Turning into a narrow street, so too can a long vehicle making a left turn. But, again, a vehicle can turn left at a much higher speed. This higher speed for an improper rolling left turn  is the principal reason there are far more collisions during left turns than during right turns: As a matter of simple math, the higher the speed, the more distance the vehicle will travel during the driver's reaction time and the vehicle's braking time.

                Particularly when long vehicles turn right into narrow cross-streets, the nose of a bus, coach or truck can remain on the wrong side of the perpendicular street for several seconds, and must often drive down it—on the wrong side of the street for 30 or 40 feet—until the vehicle can swing back into its proper lane on the right side of the street. But this can happen even with left turns, and often do when the streets long vehicles turn into have two or fewer lanes in each direction.

                The dimensions of the intersection also comprise a factor. When a bus, large truck or motorcoach makes a square left turn into a roadway too narrow to accommodate it, its nose will crash into an object—a store front, a utility pole, etc.—beyond the far-side curb-line of the intersecting street. So to avoid this, drivers of large vehicles "cheat," beginning their turns long before their vehicles' rear axles rear the extended curb-line of the near-side of the intersection. But not having to make a square turn (which, again, requires the vehicle to stop or slow to a crawl), long vehicles tend to race or roll through these turns. So by the time the driver spots a problem, it is too late to stop to avoid striking it. This "problem" is often a pedestrian in a crosswalk.

                The geometry of a right turn compared to a left turn is yet another factor: Left turns at least give a bus or motorcoach some space. Right turns never give a long vehicle the space to turn into the rightmost lane of the perpendicular street. In fact, unless the right turn is onto a road with at least three lanes to the right, a long vehicle cannot even remain on the same side of the street of the street into which it is turning as it begins its approach into the perpendicular lane. Compared to right turns, most left turns are almost sweeping. Yet the reality is that far more bus and motorcoach accidents occur during left turns  (see section below on Road Design).

                Two additional factors illustrate why there are so many turning accidents with large and/or long vehicles. One is that braking distance varies exponentially with increases in speed. The second is that large vehicles almost always have pneumatic (or "air") brakes -- and another 1/2 second of reaction time is consumed as the air in the brake cylinders compresses before it can push the brake shoes against the "drive" axle's brake drums.

                Still other factors affect turning. One factor involves how a large vehicle's driver should move his head around before and during a turn -- moving it forward, backwards, and from side-to-side (referred to by transportation professionals as "rock -and-roll").

                Finally, one major factor that contributes significantly to turning accidents is the tightness of so many schedules—especially fixed route transit schedules. The relationship between tight schedules and the frequency of left-turning accidents was finally acknowledged by a transit agency (Seattle METRO) in 2015 (“Metro buses: Pedestrians in a blind spot?” Crosscut).[1] 

Time, Space & Tight Schedules

                Because we initially think of turns into streets largely in spatial terms, we often underestimate the importance of the temporal factors. Far less attention is paid to factors like tight schedules. Some transit systems' schedules are so tight that drivers routinely turn into crosswalks with pedestrians still in them. They simply hope the pedestrians will hurry along so that their vehicles turn behind pedestrians crossing left-to-right across their windshields, or their vehicles turn in front of pedestrians crossing right-to-left across their windshields.

                Instead, this practice comprises an outrageous "safety compromise." (See Safety Compromises.) And one is likely to find it on systems whose vehicles make other safety compromises. Despite their commonly-tight schedules and multiple safety compromises, transit buses are not the only vehicles involved in turning accidents. TA President Ned Einstein has been involved in cases where schoolbuses, motorcoaches and even paratransit van- and minibus-conversions have mowed down pedestrians in crosswalks while turning left. Curiously, schoolbus and motorcoach schedules are not usually so tight that a safety compromise like making a rolling turn would often be attempted. In contrast, for a large variety of more complex reasons, paratransit and non-emergency medical transportation (NEMT) schedules are almost universally too tight.

Turning Accidents and Road Design: Safety Challenges and Possible Solutions

                Another problem relevant to both left- and right-turning accidents is that the traffic engineer or his or her counterpart in a city or state may have placed the typically white "limit line" too close to the intersection. At un-signalized intersections (particularly where sightlines on both sides of a vehicle approaching the intersection are compromised within a few hundred feet of the intersection), it may be necessary for the limit line to be placed close to the intersection. Particularly with a automobile or "Type C" schoolbus (the ones with the hood), the vehicle's hood must protrude several feet forward into the intersection before the driver's seat moves into position where the driver can actually see oncoming vehicles to his or her left or right. But this is not an issue at signalized intersections.

                While most turning accidents involve left turns, right turns involve a different set of problems. One might at first think that the challenges are actually greater than those faced by drivers in left-turning accidents:

  • The faux-blind spots (when drivers fail to "rock-and-roll") lie further away from the driver's eyes.
  • The vehicle usually spends much more time in the oncoming lane of the street into which the driver is turning (see "Dancing in the Oncoming Lane").
  • The vehicle usually travels a considerable distance, near the end of its turn, on the wrong side of the street.

                At the same time, the turning speed—or more properly, speeds of the various segments of the turn—are necessarily much slower for right turns, since the arc of the turn is much sharper (and speeding around it could literally cause a roll-over). Regardless, the very challenges of a right turn—especially the need to often spend a few seconds and the first 30 to 50 feet at the end of the turn on the wrong side of the street—makes drivers somewhat cautious. Knowing that one's large vehicle is almost certain to spend a few moments on the wrong side of the road (with oncoming traffic approaching it, and/or often vehicles poised at a limit line too close to the corner) keeps most right-turning drivers cautious and focused. In contrast, the large arcs of left turns, the much wider space for the turns, and the faux-blind spots much closer to the drivers' eyes (and even easier to look around) do not evoke the same degree of caution.

                Another distinction is that, in making a right turn, the bus driver need worry only about (a) vehicles approaching from the left after passing through a red or amber light, (b) oncoming vehicles turning left in front of it (or into its left side), (c) vehicles poised too close to the limit line in the oncoming lanes of the perpendicular roadway, and (d) vehicles approaching the intersection from the right. In contrast, a left-turning vehicle has to worry about (a) vehicles coming from the right (against the light), (b) vehicles turning right from the oncoming lane, and (c) vehicles in the oncoming lane traveling straight through the intersection—on the same green light the bus driver has. And, again, left turns not properly made often involve the vehicle traveling at much higher speeds than in right turns. So if left turns are beginning to sound complex and challenging, this is because they are.

                Another obvious problem with left turns is that the turning vehicle must first find a gap in the perpendicular stream of traffic coming from its left -- and then cross that stream in order to enter a gap in traffic also coming toward it from the right. In contrast, a right-turning vehicle need merely shoot into a gap coming from its left side, although as a trade-off, the right-turning vehicle must almost necessarily spend some time on the wrong side of the road it is turning into. (Left-turning long vehicles only have this problem where the perpendicular roadway is narrow.) As a safety matter for many otherwise left turns, particularly during heavy periods of traffic, it would be far more safe to turn right, take the first left turn, drive around the block, and finally make another right turn onto the original roadway one was about to make the left turn onto. This is somewhat less important a tradeoff in a small vehicle which can zip across the roadway, and which does not have a tight schedule whereby the regular failure to comply with it could threaten the driver's continued employment. Just the same, left turns are full of safety challenges.

                A few communities have recognized the challenges of left turns. Most famous for this is Detroit, where left turns across major arterial streets are not only illegal, but right-turn "roundabouts" have been integrated into the roadway design: The motorist turns right (far more safely) and, half a block later, turns left between two wide traffic islands, and soon after, simply turns left into the lanes of traffic traveling in the direction the driver wanted to go in the first place. Another approach -- literally left over from the horse-and-carriage days -- are "roundabouts," many of which still exist in New England states.

                Left turns also trouble traffic engineers—for non-safety-related reasons. Some of the efforts to make left turns more safe increase traffic congestion. So too do cautious drivers who may wait incessantly for a safe opportunity to turn left. Sometimes the answer to these problems is to elongate the length of certain parts of the traffic signal cycle, or to add a "left-turn phase" (or left-turn arrow) to the light cycle. These changes lengthen the entire signal cycle, and add a proportional amount of delay into the traffic flow—adding considerable time to a trip involving multiple intersections with longer signal cycles. Such enigmas are literally built into public transportation services whose routes have schedules already too tight without these safety-related delays. When schedules are tight, the pressure to take risks increases.

                When one considers all these variables, there is a reason bus, truck and motorcoach drivers typically receive six weeks or so of training. Unfortunately, this training has no meaning if it is not (a) understood, (b) retained and (c) applied. When it is not, the risks of turning accidents increase almost exponentially. But as noted, many, many factors beyond negligent training contribute to turning accidents and incidents.

 

[1] That article also blamed these accidents largely on "blind spots." Of course, as noted, if a driver properly "rocked-and-rolled" to see around window posts, exterior rear-view mirrors and possible other appendages or windshield blockages, there would be no blind spots.