I can't find any factual stuff about whether and how often flashing bike lights cause seizures.
In an epilepsy organization's document, I see
"Red flashing bicycle lights (light emitting diodes, or LEDS) have triggered seizures in a small number of people. This has happened when they were very close to the lights, setting them up."
http://www.nhs.uk/ipgmedia/national/epilepsy%20action/assets/photosensitiveepilepsy.pdf
Based on the above, that organization asked cyclists (in the UK) to stop using flashing lights.
I think if bike lights were really causing lots of, or even some, seizures out there on the roads, there would be more reports and data.