My first year of commuting was on a bike that has cheap hub-powered lights with no flash mode.

My 2nd bike didn't come with lights so I got a set of Lezyne USB-charged lights, which do have flash modes on both head and tail. I've noticed drivers hesitating to pull in front of me with this setup, whereas when I've ridden the other bike, they just pull in front like I'm not even there. I generally prefer riding on the street over the sidewalk, but I do have to ride on sidewalks/MUPs that cross streets and parking lot entrances for small segments of my commute, in order to access bridges.

Not sure if there's a difference between slow and fast flash modes.

I have a colleague who doubles up on his lights - two in the front, two in the back.