For simplicity's sake let's concentrate on a one loop bike course (not very many of those left). Now take 2,500 competitors, or about 100 less than the most recent Ironman Canada (where there is a one loop course).
2,500 competitors all riding "legally" would take up 22,500 meters or 22.5 km on the road. That is 14 miles of roadway, all filled with racers riding just within the legal boundaries allowed. Assuming the first competitors left the water at 55 minutes, went through transition in 5 minutes and rode 28 mph, that would mean the last competitor could exit the water at 1hr25, take a 5 minute transition and get on their bike.
The whole road would then be filled with a continuous 14 mile train of riders.
The reality of course is a bit different. People do not come out of the water one by one with a nice and constant spacing. At the most recent IM Canada, 2,250 people exited in the first 90 minutes, but they were far from evenly spaced. 1,000 came out at between 1 hr 15 and 1 hr 30. If you study the data you can find many such clusters.
The clustering is part of any North America IM race. 1,000 or more racers enter the bike leg in a space of 15 minutes or less. 1,000 racers need 9 Km (or 5.6 miles) of roadway to sort themselves out. One has to ride 22.4 mph to cover that distance in 15 minutes. Clearly, unless many are forced to wait, it is not possible for all these people to fit in properly and obey the strict no drafting rules set up by the race directors. They simply can't do it.
Indeed what we see in a race is that most mid-packers ride in dense groups. They do not necessarily do so because they want to cheat -although some undoubtedly do- but because they have no alternative. They cannot physically fit into the space available without wasting valuable time. And so we have something analogous to a typical "California freeway" situation.
In California, nearly everyone drives faster than the speed limit, but only a few unlucky souls get tickets. It is not a good situation as it allows the CHP to pick and chose whom to punish. The same, unfortunately, appears to be true for Ironman races. The referees can arbitrarily pick people to give penalties too.
If you are a slow swimmer and a fast biker, you are very likely to get such an undeserved penalty. In Canada I got a "yellow card" for riding over the centerline while trying to pass a group of riders. I did not intend to cross the centerline, but the racers in front of me slowed down on the hill and I had to swerve to avoid crashing into them.
Never mind the fact that some Ironman races are run on open roads and that the rules then specify that the drafting zone behind a car is 35 m (IM rule book). If more than a few cars are on the road, we'd have to stop the race repeatedly to make it legit. In Canada for example, there are often long lines of cars on the "backside" of Richter pass, all trying to get past groups of riders. Many cars are filled with spectators and family members, but there are also trucks and other vehicles on the road. These vehicles are often backed up and forced to drive slowly in long lines.
Ran 7 miles in the hills today. Yesterday I rode on rollers for 1.25 hrs. On Saturday, I ran 6.25 in the hills.