Bike lights: people think you need bright lights for darker areas, and little or nothing for well-lit cities. The opposite is true. In truly dark rural areas, where your pupils can dilate, a little light goes a long way. Mixing it up with motor traffic in town means you need car-bright lights (>50 lux) just to be seen reliably. Exception for the dark sticks is if you're riding fast (long downhills): then you need a beam you won't outrun.
We're great believers in bright lights, as vastly more effective than hi-vis "avid cyclist!" uniforms, and more significant to safety overall than body armor (crash helmets). Harm prevention beats mitigation. Of course, maxing everything out is safest of all, but as a matter of budgetary priority, we say lighting should come first. This is also opposite common priorities.
Generator lighting systems, built around dynamo hubs and permanently wired LED lamps fore and aft, are the gold standard. They are now plenty bright enough to be conspicuous in broad daylight, and efficient enough to add only imperceptible drag.
There's a reason it's the law most places that motorcycle headlamps must shine full-time: daytime running lights save the lives of people drivers otherwise too often say they "didn't see." The technology is in place for similar logic to apply to bicycles. Popular opinion and culture lag behind logic and technology.
To be meaningful for daytime use, you need the very brightest lights of all. A note about brightness: most bike lights sold in the US (generally battery/USB) cite lumens. Lights sold where people are more serious about bikes as transportation (generally dynamo) cite lux. This makes it not possible to compare brightnesses directly. Most makers citing lumens don't want to talk lux because their products compare poorly by that metric. They might not even bother to measure. But lux corresponds a lot more closely to the plain meaning of brightness than lumens.
A candle produces almost 13 lumens. Not very bright, right? But if you gather all the light a candle throws in all directions, and concentrate it in a narrow beam as by mirrors and lenses, it seems a lot brighter. That illumined area can be described in terms of lux: it's the density of light in a given area. That same strategy of concentrating the light where you need it can keep the light out of other road users' eyes, on the road instead. Lights rated in lux tend to do just that, while lumens-rated lights usually scatter the light over a broader area, less subjectively bright for a given power/efficiency level.
Who needs light thrown over a wide area? Nighttime mountain bikers, because then it's just as important to see the branch that's going to poke your eye out as the next stump to jump. Mountain biking is recreation, not the urban transport our shop focuses on. In Portland, a hypothetical night MTB ride is likely to begin and end by driving a car with headlamps rated in the neighborhood of 100 lux. Nobody cares how many lumens that might be.
Unlike battery lights, with generator lights you need never worry about state of charge. You can't forget or lose them. They're unlikely to be stolen from your parked bike. Battery lights bright enough for meaningful daytime use require almost nightly recharging absent batteries heavier than whole generator lighting systems, such as come on electric assist bikes. Almost nobody who uses today's dynamo lights will choose battery lights tomorrow.
Retrofit to bikes lacking them, generator lighting costs start around 3x a decent helmet. We wish more transportation-focused bike manufacturers would build them in at the factory, as most cost-effective. Breezer has long been a leader in this regard, with exceptionally well-lit bikes under $800, under $700 come 2018 [edit: Breezer just canceled the sub-$700 2018 dynamo-lit Downtown 8+ because ... I'm so cynical]. Above $1000 without built-in lights? That's outrageous. And still too common.