I had an interesting encounter with an instructor about 18 months ago. I was approaching a junction with a major road, signalling to turn left, when I noticed that a learner driver on my right, on the main road, had slowed right down. I stopped (could have continued, as there was a cycle lane), waved him through, and called out "Don't be afraid, I'm only a cyclist!". About 200 metres further on, I joined a queue of cars, stopping to (politely) inform a driver that his nearside brake light wasn't working. Then I started filtering, on the left. The next car was the learner. The instructor had her window down, and loudly berated me for making an obscene gesture and swearing at her learner. Unusually calmly, I explained that I'd noticed the learner appeared nervous, and had used a signal which would be instantly recognised if used by a PC directing traffic, and asked her how on earth she could construe it as obscene. I also repeated what I had called out, and asked whether she'd deliberately mis-heard me because I was on a bike. Then I asked her if she'd noticed me stop and have a word with the driver behind her - she had - and told her what I'd said to him, and how he was grateful for the info. That's one instructor cured of her prejudice about cyclists; we've had a few friendly conversations since then.