One of the frequently uttered complaints about cyclists made by the non-cycling or even, infrequently cycling public, concerns the amount of space the peloton takes up on the road - you know, things like "why can't those damned cyclists ride in single-file, etc. These are largely the utterances of people who have been conditioned by their auto-dependence, where single-file procession is a fact of the activity. Sure you can change lanes, but really that is just moving from one line to another. The peloton operates under entirely different conditions. These conditions are influenced by 1. physical features in the environment, 2. the competitive nature of the activity, and 3. vagaries inherent in any large group of individuals.
Take relatively minor things in the physical environment - cracks, rocks, glass, nails and other debris - these are things too small to even be noticed by drivers, let alone be of concern, but can be serious obstacles to a rider, capable of knocking a bike off course at the least, and laying it flat at the worst. Obstacles such as these will require an avoidance action on the part of the rider, anything from briefly lifting the front wheel, to bunny-hopping the offending object, to swerving around it. When a rider is pedaling along solo, there is usually little problem; but when riding in a group the reaction is compounded. We are all somewhat familiar with the saying "for every action, there is an equal and opposite reaction"; you may have first heard this in high school science class. If one rider in the middle of the peloton moves left to avoid a pothole, for instance, the movement may put him into a space already occupied (which is of course another rule of science, that no two objects can occupy the same space at the same time) by another rider, thus forcing that rider to move left in turn. This action and reaction may be repeated any number of times until reaching the rider on the outside of the peloton.
The action of slowing will cause similar reactions and, if the slowing happens suddenly, riders especially those riding along the sides, will often move even further out, provided space to do so is available. Consider a barrel-type spring for a moment - a spring, when under load will be narrower at the center as it stretches out to hold the weight; remove the weight and the spring will contract, widening out at the center until it returns to its non-load bearing shape. The peloton operates much the same, though for a different reason. As the speed of the peloton increases the group tends to become stretched out. One of my few moments of glory came in a local road race when I accelerated fast enough and long enough to look back and see one long line stretched out in my wake. Of course that speed cannot be maintained indefinitely, and as the speed is removed (just as the weight of the spring) the peloton will bunch up and spread out at the middle.
"Sommers, hold your line" is perhaps the most oft quoted line from the most well-known cycling film, Breaking Away. Of course "line" in this case does not refer to an actual line persay, as if the race were proceeding along in nice ordered rows. No, what it refers to is following a predicable course and not making any sudden or unexpected moves necessitating a contrary reaction from other riders. I have often thought that, when viewed from above, the peloton must resemble a single living organism, such as an earthworm, that expands and contracts as it moves along. The problem is that this resemblance is one of appearance only. Since the peloton is composed of many individuals of differing abilities it cannot be expected to operate in an entirely predictable manner. On any given day, on any given group ride, the riders in the peloton will have a wide range of physical ability, practical experience, and what I term "comfort level". These vagaries and differences can have as much effect on the entire group as do physical features encountered along the route. Outsiders looking in will often wonder at how close the riders are to one another. Not infrequently this proximity results in harmless bumping, and yes, even crashing in the middle of the peloton. Newer riders, lacking in experience or the fortitude to ride in the middle of such a seething mass, will often ride to the sides of the group. This can be safer, but is also more difficult and tends to further spread out the group.
Cycling is a competitive activity, and most group training rides effectively simulate race conditions with riders doing what is necessary to succeed, within reason. During any training ride there will thus be occasions when the peloton becomes strung out in a long line due to increased speed, and there will be other times when the speed slows and riders bunch together to conserve energy. Single-file does not happen during a race, unless there is a team time trial taking place, or a smaller group of riders gets away from the larger group and forms a paceline to maximize their efforts. Expecting that a group of riders will follow one another in an orderly single-file line, and confining themselves to a narrow ribbon of asphalt along the margin of a road or street under these conditions is anything from optimistic to delusional, depending upon perspective.
