There are two routes Cyclist can take from St. Austell to Pentewan. The first is one of the nicest cycle paths I have ever seen, which runs for about 4 miles through ancient woodland beside a river and is a wonderfull experience.
The other is the main road from St. Austell to Mevagissey. It is very bendy and has few overtaking spots and runs along a similar route.
So it baffles me that so many cyclists choose to use the main road and sit choking in the fumes from the traffic jam that inevitably builds up behind them. Particularly when a far better option is available.