Yes-ish
it depends…
let me explain, I have a 3 bike stable, and depending on where and when I’m riding will determine if a helmet is needed…
Bike one; an old rigid steel Fila Taos MTB converted to a roadgoing singlespeed, slicks instead of knobbies, and a Surly Singleator chain tensioner, this bike is ridden around town (Dover, NH) at work, when I go out to lunch, I typically hop on the bike, it’s about a mile or so round-trip to the furthest away place, top speed is around 15 MPH, average is 8… this bike I don’t wear a helmet on
Bike two; a Trek 4500 MTB, currently with slicks, one of my road-going bikes, for my easier recreational rides, on the back roads around the house, sometimes I swap out the slicks for knobbies and do some light offroading, top speed is around 20 MPH on road, average is 8, this bike I sometimes wear a helmet on, but it’s not mandatory
Bike three; a Giant FCR-3 flat-bar road bike with Shimano SPD flip-flop pedals (SPD cleats on one side, platforms on the other), this bike is my highest-performance bike, top speed is 28-30 MPH, average is 10-12 (on the pedal flats), and I’m typically on it for 5+ miles on average, this one I DO wear a helmet on, as I’m moving pretty quickly for a “Clydestale”…
the Giant sees the most road time, the Trek sees the least (I’m trying to get one of my work neighbors to buy it off me)