Since we don't have any suggestions yet, I'll bring up a post from a few weeks ago. I don't own a Cygolite headlight myself, but I ride with someone who does, and I'm impressed. At circa $50 they provide a flash mode that is very visible in full sunlight -- they brand it as "DayLightning™" and says it's brighter than the high steady mode on the same light.
If that's in the range of what you're willing to spend, this is a worthwhile fulfillment for your visibility objective. (But I don't have anything for you on your objective of having a headlight automatically turn on upon detecting motion and turning off after detecting lack of motion.)
At $50 the Cygolite Metro 360 USB Bicycle Headlight is an attractive option. 4.6-star rated.
Upgraded for 2014, the Metro 360 features 6 modes, new custom TIR optics and increased output that lights up the demanding roads with its powerful 360 lumen 4 watt LED. Recharge conveniently through a USB port to save you the cost of replacing batteries ride after ride. New 24/7 Safety Technology features night and day specific modes for added safety: 2-in-1 Steady Pulse illumination alerts motorists with flashes while constantly lighting your path while new DayLightning mode boosts 500 lumens of lightning like flashes to demand motorist's attention during the day. Compact, rechargeable and super bright, the Metro 360 is geared for any commute.
The
Metro 400 appears to be the current model at this performance point. It's available at
$55 at the same site. It's 5-star rated but with just 3 reviews so far.
Metro 400 Product Description- 400 lumen output with a 4 watt Cree X-Lamp LED
- DayLightning flash mode boosts 500+ lumens of lightning-like flashes for added day riding safety
- Custom TIR optics maximizes beam angle and throw
- Water resistant design for all riding conditions
- 1:45-12 hr run time
- 6 lighting modes: Bicycle: High, Medium, Low, SteadyPulse, DayLightning, and special: Walking
- USB rechargeable internal Li-ion battery with 5hr charge time
- 24/7 Safety Technology features SteadyPulse to alert night time motorists with pulses while constantly lighting your path, and DayLightning for boosted output in lightning-like flashes to enhance rider visibility in peak daylight
- Includes handlebar mount
- 110g