In heavy overcast, or close to dawn or dusk, lights are probably a good idea.

Bright sunshine? Bah, why bother? If you're worried about visibility, go for bright jerseys, jackets, or vests.

I haven't repeated this story for a year or two, which supports my point. Some years ago I was on a group tour. One rider had on bright green rain suit. I saw him as soon as I came around the curve at the bottom of a long hill, maybe a quarter mile away from him. I was riding slightly faster than he, so I slowly caught up. I was within perhaps 100 yards when I started seeing his seven (!) PB Super Flashes, all pulsing. Now some will point out that the newer versions put out twice as much light. Applying 1/r^2, I should have been able to see him from 144 yards back; still much nearer than the quarter mile away when the brightly colored garments caught my eye, under a tree canopy, and overcast and drizzling.

Go for the bright colors in your clothing, instead.

Another vote for the Cygolite hotshot 150. Great battery life, options for flash pattern/brightness, weatherproof, and very visible in full sun.

I do NOT KNOW... BUT, when I put my lights on (flashing front and flashing rear,) I did seem to notice, some difference. People passing me, were passing me farther, away, and people pulling out in front of me stopped pulling out in front of me as much... JMO as I think about the differences since I put those lights on... again JMO...

I want to be visible so yes flashing white in front and flashing red in back. The difference in visibility is huge.

I also drive with my lights on (not just DRL) at all times

I can see for myself the how much easier it see cyclist with lights and cars even in bright son with lights and on gray days bikes disappear

I don't think they are necessary for daytime but I do use them on the road.

Cygolite metro 550 on the front and cygolite hotshot 150 most of the time.

I am not trying to be a smart Alec but I was wondering why you run your headlights on your car with daytime running lights ?

good question, and not smart alec.......becuase I want the tail lights on also for that visibility.

driving with lights on was something my dad insisted on when I learned to drive in montana......you could see cars with headlights on literally miles earlier in dayight in many cases than you could cars with headlights off. Makes a difference in places where you have lot's of 2 lane roads and have to move to the other lane to pass

squirtdad thanks for the explanation. My brother in law used too but I am not sure if he still does run his lights all the time.

I never ride without a light set up....ever.

You can never know if your taillight works (to prevent a collision). You can only know if it does not work.

I use one pretty much all the time. I don't know how much difference it makes. But the cost is so low to own one and run it that it seems like good sense. I also use a headlight whenever I ride, for all the same reasons.

I think it works. A co-worker driving into work said he saw me 1/4 mi away.

Well I guess that all depends if you think distracted driving is a problem.

It's very common around here in the Netherlands for cars to ride with their headlights on. Much better visibility than daylight running lights though modern LED lights have mostly solved that issue.
In Scandinavia cars are required to do so at all times and most cars cannot even switch them off.

I remember riding in Southern Spain during summer with headlights on and other drivers signaling me my lights were on so it was definitely less common around there.

As such I can imagine it being rare if you go even further down south like California. Keep in mind that everything below Southern Spain is basically Northern Africa and the Sahara. And that is the same latitude as any state in the US below Oregon or Ohio.

I drove in North Carolina last year and found drivers to have really weird customs when it comes to lights anyway. Like turning on your fog lights in a heavy downpour (which will only blind people, as such it is prohibited in most of Europe) or turning on their alarm lights in the same conditions while still doing 60+ MPH. Is it preventive lights in case they crash?
And cars without rear window wipers.

Personally, not only do I think it is, I know it is.

Think it really depends.

My commute used to be 9 miles of MUP bookended by roughly 0.5 miles of residential. I always turned on my light to the lowest flash or slow strobe or even steady so as not to blind other MUP users behind me, but it was far more important to have a headlight on the street since cars are going only slightly faster than me and have plenty of time to see me, however the same couldn’t be said about those turning into my path in front of me.

I also commute now by bike to multiple places for work, and in those more traffic heavy roads I definitely use a bright but slow flash still, I actually find drivers give me more room. As a driver I think the mentality here is that if it’s a crazy flash, no one wants to stay behind you for long and drivers can tend to get annoyed/aggressive.

if I’m riding in a peloton or group ride then it’s low slow strobe or nothing at all, the pack itself is visible to vehicles.

I don't think active epileptics are allowed to drive.

Been using front and rear lights for the last 3 years. Never ride without them.

Yeah, we're more worried about the drunks and texters. Let's not buy trouble.

Although technically if you are drunk or texting you are also not allowed to drive.

Some people ride in city traffic. Other people ride on off road trails. Those on off road trails don't need lights in the day time.

And some people ride in city traffic to get to off road trails.

I never run lights during the day and don't use blinking lights ever because they are obnoxious. Several of my bikes have dynamo lighting so I just have to flip a switch for the lights to come on, but I leave them off because they are unnecessary. On my one mile commute to the train station I don't use lights at all. I just wear one of those reflective vests like construction workers wear. Don't pass trucks or buses on the right and you'll be fine.

Tail lights: I say "yes" necessary if only because commuting, we can get caught riding twilight or dark working late, a forgotten meeting, errands, or unusual emergency. We don't necessarily need them on at all times (but why not anyway?)

For commuting where I'm sharing a lane with a 2000+ lbs vehicle, a bright flashing red light in the rear is a requirement. I don't want some driver roll up and over me because they couldn't "SEE" me, especially during the early morning hours when cager zombies are still waking up and there's heavy early morning fog. My commute (when I was doing it post kung flu) was partially on main roads, with some residential and MUT mixed in. The road portions always had commuters, buses and large commercial trucks. I feel more at ease with a red blinky light protecting my backside.

Forgot to mention, now that we're on a work from home, I recently transferred a red blinky light onto my mountain bike so I could get to the local trail which is 10 miles from home with a small portion of that on regular streets. Parks are officially closed meaning you can't drive to recreate there but it's allowed if you can get there by foot or bike. I turn it off when I'm riding the trails and turn it back on for the trip back home.

A PBSF was the go-to choice 10 years ago. Now I use much brighter lights, front and rear, during the day. I use flash mode, the slowest setting on lights with multiple flash modes. With so many distracted drivers nowadays it's no guarantee they'll see you even with daytime flashers.