Lights and aero dynamics
 

Do lights affect aerodynamic that much or nah?

Everything affects aerodynamics. It just depends on how fussy you want to be about it. Just like helmets, some lights some have more drag and others less drag. But still you need a helmet and if you ride at night or where lights are required during the daytime, then you have to have them.

Minimal amount.

If in front of your headtube, less than if on the end of a skewer. Taillight behind saddle, not much.

Just under 0.001 m^2, probably 0.0008 ish by itself. At least an order of magnitude less than shaving. You do shave your legs?

The lamp aero effect could be larger depending on the placement

It doesn't matter. You are unlikely to have lights on your bike while racing and if so you are likely in an MTB or gravel race. If you are on a road bike riding and need lights you need good lights with a good beam pattern and brightness and worrying about aerodynamics over safety is not the way to go.

One also wonders, if it is dark enough to need lights ... like, how fast do you really want to be going? As speed decreases, so does the aerodynamic drag of the bike and it's accessories.

In the UK it is compulsory to run front and rear lights for all time trials (there are no rules on the size or brightness though).

Aerocoach actually did some testing and found a "sensible" front light made pretty much no difference to your overall aerodynamics, we're talking less than 2 watts at 45km/hr (28mph).

https://www.aero-coach.co.uk/front-light-aerodynamics

Depends. Are they 30 watt lights or 60 watt lights?

Tail light will make you faster if it’s red. If it’s blinking then it only helps you half the time.

I run lights whenever I ride my bike, day or night. I often have more than just one headlight and one headlight. One day, having my lights might save my life. For all I know, it already has. There’s no way of knowing for sure if lights increase my safety, but I will be able to know if they did not help, i.e. if I get hit.

My lights don’t slow me down much. If they are the strap-on type, the strapping on and off slows me down more than the air drag does. I have a couple of bikes with hub dynamos, and they create a little bit of drag, but it’s not very much, and it’s worthwhile for me since I never have to strap on the lights nor charge them. Aerodynamic drag is relevant only when you’re riding very fast, and even then, it isn’t a burden unless you are highly competitive.