In my experience, smooth or treadless tyres are perfectly good in the dry, and on smooth/good roads.

Smooth tyres are adequate on wet smooth roads.

Wet AND oily roads, or wet AND gritty roads is where tread starts to becomes useful. When the road surface is not firm and cohesive, tread allows spaces for those particles to be such that they're not a ball-bearing.

Have you seen occasional islands or moraines of fine dirt or sand or stones that accumulate where cars don't drive? In the corners of intersections and between lanes.

^{Example island of loose gravel, in thew middle of the green cycle lane.
Fine to roll over, but if you were hard-braking for the red light then it might be different.}

These are okay to ride straight over, but exacerbates slidiness when turning or manoevering or braking. Tread may reduce the "ball bearing" effect of turning while on loose rolly gravel. If its hard-packed like cement then its just bumpy and not relevant here.

Ice is another matter completely - tread helps not-at-all when turning on hard ice. In this case you want spiked tyres.

Snow - I can't speak for riding in snow etc, because there's not enough of it here for me to have experience.

In my experience, smooth or treadless tyres are perfectly good in the dry, and on smooth/good roads.

Smooth tyres are adequate on wet smooth roads.

Wet AND oily roads, or wet AND gritty roads is where tread starts to becomes useful. When the road surface is not firm and cohesive, tread allows spaces for those particles to be such that they're not a ball-bearing.

Have you seen occasional islands or moraines of fine dirt or sand or stones that accumulate where cars don't drive? In the corners of intersections and between lanes.

^{Example island of loose gravel, in thew middle of the green cycle lane.
Fine to roll over, but if you were hard-braking for the red light then it might be different.}

These are okay to ride straight over, but exacerbates slidiness when turning or manoevering or braking. Tread may reduce the "ball bearing" effect of turning while on loose rolly gravel. If its hard-packed like cement then its just bumpy and not relevant here.

Ice is another matter completely - tread helps not-at-all when turning on hard ice. In this case you want spiked tyres.

Snow - I can't speak for riding in snow etc, because there's not enough of it here for me to have experience.

The two main reasons for changing a worn tyre are:

1. if there's a sudden increase in the frequency of punctures (this means the rubber is too thin for your riding conditions)

2. there's damage where you can

   • see the tube inside at any time, or
   • see a lot of threads, or
   • if the tyre is bulging in weird ways.
     In these cases, you're asking for a blowout which is never wise. Booting may help, but only to get you home.

Personally I'd ride either pictured tyre until the frequency of punctures increases or significant damage happens to the tyre.