Frequent causes of a solo crash is from debris hazards.
The movement of automobile traffic tends to “sweep” debris off the roadway and onto the shoulder. Debris in the form of sand and gravel, and leaves, can be dangerous for many typical road bicycles. Car parts, and things that fall off of and out of motor vehicles are also deposited on shoulders and must be avoided. Trash and broken glass are commonly present.
Other frequent causes of solo crashes are road surface damage, like cracks and ridges parallel to the direction of traffic, and potholes.
Often, when a roadway is re-surfaced, the shoulder itself is left alone as a cost saving measure. Because shoulders are not designed as a normal travel lane, they get less wear than the roadway. And they get less attention when they deteriorate as well. Potholes, when they appear, remain longer before they are attended to.
A partial re-surfacing of the highway will often leave a lip at the transition between the old surface of the shoulder and the new surface of the roadway. If struck at an oblique angel by a bicycle, the tire may not find enough grip to climb the lip, causing a diversion fall. (The bicycle tire would go to the right, and the cyclist, following the laws of physics, would continue straight and off the bicycle.)
All of these hazards require a cyclist to expend a lot of energy and attention scanning for a clear path on the shoulder. Energy and attention that can be used elsewhere to good effect if the cyclist uses the roadway without all these dangers.
Shoulders vary in width, sometimes disappearing entirely without notice, either to a shoulder user or to overtaking traffic.
A primary function of a shoulder is to help preserve the roadbed from erosion. Any extension of an improved shoulder beyond a few feet is to provide for a benefit other than erosion protection, as discussed below. These other functions are secondary to protecting the roadbed, and so are sometimes dropped as the highway proceeds.
Disappearing shoulders commonly occur when the roadway becomes elevated and a guardrail is installed on part of the shoulder space. Bridges often require a reduction of a shoulder. Right turn only lanes are often built on a shoulder.
These reductions in shoulder width are nearly always done without an announcement to travelers. A cyclist suddenly needing to merge onto the roadway can come quite unexpectedly to a distracted motorist.
Better sight-lines is another purpose of shoulders.
Shoulders provide better sight-lines for traffic about to enter or cross the road.
This is especially important for cyclists who are riding in a visually cluttered environment, where there are a lot of driveways, signs and businesses, with a lot of turning traffic. Those are bad places to give away this advantage afforded by shoulders.
Shoulders, by their nature, continue to the right of right turn only lanes, placing a through cyclist in conflict with right turning traffic.
A lawful cyclist driving on the shoulder, will place himself in a dangerous position if he follows the shoulder through the intersection. He is in danger of being right hooked, unnoticed by oncoming left turning traffic, crossing traffic from both his left and right, and traffic turning onto the road in his direction of travel.
It is illegal for him to use the right turn lane if he intends to go straight, but he somewhat lessens the hazards noted above, provided other drivers don’t assume he is turning right.
He needs to merge onto the roadway prior to the beginning of the right turn only lane. Which by itself is no real burden. But when faced with a dozen or so of these merges back and forth between the shoulder and the travel lanes, as would be necessary in some areas, it can become quite taxing.
Shoulders provide parking, forcing a cyclist to merge onto the roadway to get around a parked vehicle.
As one would find necessary to avoid the door zone, the cyclist should give the parked vehicle a lot of room.
Someone working under the hood is not likely to watch for or expect a cyclist when he rounds the front of his car, a hazard the cyclist must be aware of when overtaking any parked vehicle.
Operating around emergency vehicles is also a watch-out condition. Motorists are distracted from the task at hand by the flashing lights, and emergency workers can suddenly step out into a cyclist’s way.
Many shoulders have rumble strips, narrowing the usable space on a shoulder for a cyclist, and providing a hazard when he needs to merge onto the roadway and when he wants to return to the shoulder.
Shoulders are outside most traffic’s scan area, making the cyclist’s presence less conspicuous.
This is most dangerous at intersections. Crossing traffic will often “shoot a gap” formed by the motorized traffic, never noticing the cyclist traveling on the shoulder, because they are only scanning the travel lanes.
Overtaking traffic will right hook cyclists on the shoulder. Distracted drivers will never even become aware of you, as they only scan, when they do, in the travel lane directly in front of them. They do not perceive what is on the shoulder, as they never intend to steer their automobile onto the shoulder.
The faster the posted speed limit the more acute this phenomena becomes, making shoulder use increasingly dangerous to this particular hazard as highway speed increases.
Because of the crown on the road designed to promote the shedding of rainwater, and the natural fear of head-on collisions, as well as the perception of clear space to the right, distracted motorists have a drift bias to the right. (For those goofy countries where they drive on the left, the drift bias is to the left.)
As the driver turns away from the task of operating their vehicle, seat of the pants corrections to their line of travel in most cases results in their vehicle drifting to, and even across, the edge line.
This phenomena of drifting off the road is so common that guardrails are erected, wide shoulders are paved, rumble strips are installed, trees and other dangerous obstacles are cleared for hundreds of feet from the edge of the road, and even parking on the shoulder is not considered safe. Think about that for a moment! (Go ahead, I’ll wait…)
“Why do motorists seem increasingly incompetent? Well maybe because the traffic engineers keep trying to design the roads to compensate for irresponsibility and stupidity. Everything and everyone must be gotten out of the way of the incompetent!
When you design the system for minimal attention to the task, guess what you get?” –Keri Caffrey, The Incompetent Shall Inherit the Roadway
More and more gadgets are being introduced to the cabin of the modern automobile. The temptation to drive while distracted is stronger all the time.
Each successful episode of driving distractedly emboldens the operator to repeat the process. Just as not hitting other things with you car is engrained through constant reinforcement, distracted driving seems less and less dangerous each time one does it successfully. That which works is repeated.
There is a big difference between being conspicuous and being noticed. Wearing bright clothes will make you more conspicuous, but nothing informs a distracted driver of a hazard that he ought to pay attention to quite as effectively as driving a bicycle smack in the middle of his line of sight.
Being where crossing traffic is looking for other traffic, like in the travel lane, can mean the difference between being noticed and just being conspicuous.
If you are not spending as much effort from avoiding debris and bad pavement on the shoulder, you will be able to give more attention to potential conflicts at intersections while in the travel lane. By remaining in the travel lane, you are not needing to merge back and forth from the shoulder, and so your line of travel is more predictable.