This is going to make everyone's eyes glaze over, I'm afraid.
There are limits to the size of the field of entrants in a race, mostly determined by bottlenecks in the width of course. Trail races usually start in an open field or on a street, giving runners a chance to spread out before they hit single-track trail, where too many runners cause people to have to wait. It's easy to find track races where they let too many start; though the course is 8 lanes wide, the effective width is only 2, and only 1 on the turns; too many runners leads to bumping and often to falls.
Road races, when they grow unwieldy, have staggered starts. The NYC Marathon, for example, starts wheelers, then elite females, then elite males, then everyone else (from two different start points). This is to cut down on accidents from crowding and, to a lesser extent, to cut down the time people have to stand around waiting to be able to run. The problem with this is: if you didn't start at the same time and run the same course, you weren't in the same race; a great example of this was the Melpomene 5K held decades ago in Minneapolis, where one year the women ran, then the men ran separately, but it rained during one of the races.
If you combine the results of multiple heats, as is done with some races, like the Meet of the Miles, you still don't get a normal distribution, but people bunch together at specific goal times.
There is an ideal race population, which is rarely achieved. The number of finishers should form a normal curve of distribution when plotted against the logarithm of time. The math of that is horrible, but fortunately the middle 25% of finish places form a straight line plotted against the logarithm of finish time. Almost no races actually fit this profile (the Brian Kraft 5K is the only one in Minnesota that regularly does), for a number of reasons. First, you have to have a lot of runners, thousands, to form a smooth distribution. Second, there can be no "ringers" brought in. Then, everyone has to be competing to run as fast as they can; this, in fact, is the major reason it doesn't work in marathons, because the majority of runners don't care when they finish, just that they finish.
Very long races have the problem that the first runners often run alone, with hours between finishers. If you can't see the runner ahead of you, you rarely are racing them. Generally, in track races, "losing contact" happens with a gap of 10 seconds. As I said in the first post in this series, several runners finishing per second can't be considered racing, either. So, ideally, you want 2-10 seconds between runners from start to finish, but this never happens. There's a "sweet spot" at about 100 runners per mile (+/- 50%), regardless of race length, where most runners will fit that criterion.
Field Size Stability
In 1999-2000, I did a survey of local races and found that those that had 500-1000 runners finishing were relatively stable. Those that had more than 2000 tended to grow until they became problematic, then suddenly fell apart (the largest races have continued adding races, often on other days - the Twin Cities Marathon now has a 10 Mile, a 10K, a 5K and shorter family events, plus lead-up races throughout the year). Races with fewer than 300 finishers tended to die out, unless they had motivated race directors.
Very small races, as they were done in the 1970's, ideally had three volunteers. One would stand at the finish reading out the times while another jotted the times down on paper. A third handed the runners their place, written on something that would stand up to weather; tongue depressors were commonly used, which are essentially large popsicle sticks. After everyone finished, the finishers would then hand their stick back to the person with the list of times, so that their name could be attached to their time. This works well for races of 50-70 runners and okay up to maybe 100.
Putting it together
With all the numbers above, one can see that races of only 0.5-2.0 miles would fit the criteria. These distances are almost never run, though it's been shown that 70% of runners' best distances for racing are under 5000 meters (which is why track races have so many sprints and few long races). These very short distances are also good entry points to road racing for beginners, particularly children.
No one will travel to go to a short race of odd mileage, so these races will be very local, with the same runners showing up repeatedly, which is what one wants for racing. Also, one can recover from these races quickly, so one can run a lot of them. The last challenge is in getting people motivated to run these and run them as fast as they can - that, I hope, I will cover in the next post.
Move it on over
1 hour ago