Unfortunately it seems to be the main problem with the AI, it would be great if anyone knows a way to improve this. I have done the best I can, by changing talent files and making separate engine & HDV files for the cars, so the back of the field is slower, and the leaders do then get past a bit quicker. I also set the AI difficulty a notch higher to make up for the fact that I will get past backmarkers quicker than the leaders, so I end up with very good racing, but it would really top this sim off if the AI would pay attention to blue flags!
Another suggestion - give the top 6 drivers the 'AI learning' for the particular track, and this will make them 2 or 3 seconds quicker than the others, at least this is what I have found, I seem to lose about 3 seconds to AI cars when they have completed AI learning.