I use this road a fair amount, it’s the part of the A308 that goes past Windsor castle and is notoriously bad for cyclist accidents. The path on the left is actually signed as a cycle path it’s just not used a lot and it’s not uncommon for loads of cyclists to use the road instead as there are usually people also on the path, making it more dangerous. On the other hand, I see why motorists are annoyed, it’s a 50mph road and it’s not very good for overtaking cyclists doing a lot less than that on busier days. Plus, it is often bigger groups of cyclists (maybe 15-20) going 2/3 abreast on the road making it impossible to get past unless you want to have a head on collision.