I'm still confused how they made it through regulation. Image you're a security-engineer and read "On average the indicator light runs at only 50% because we think it looks cool."
You're right. I imagined that the light would start to fill up immediately after turning off but it looks like thats not how it works, it really is completely off for some time.
Besides the fact that they're blinking lights so only on 50% of the time by design, consider the fact that regulations specify the minimum luminous output which these cars obviously pass. Add to that the fact that you're using modern high power LED lights and that the human eye perceives light in a logarithmic manner, it wouldn't surprise me if these indicators will be more noticeable on their "minimum" output than their old incandescent bulb counterparts.
Have you measeured their area? Betcha they're at least twice as big as the smallest ones on the market -- whichever vehicle those are on -- so 50% of these is still more visible lit-up area than 100% of those.