On which weather conditions should I choose my bicycle with studded/spike tyres? I've recently bought a new bicycle for my daily commuting and fitted my old bicycle with studded/spike tires as a dedicated winter commuting bicycle. (They're Schwalbe "Winter" tyres, not "MarathonWinter" as I initially wrote on Twitter. I'd have bought MarathonWinter, but they're not available in 16".)

Now I wonder when I should choose which bicycle (both are Bromptons), especially under which weather conditions?

I've seen and read When are studded tires necessary?, but on the one hand the question's focus is a little different and on the other hand, none of the answers really answers my question.

A little background:


Two years ago I had a self-caused bicycle accident on a dry and sunny winter day due to snow melting, flowing over the street and there freezing immediately again.
I've thought about using studded/spike tyres for quite a while and after that accident I'm sure that there are cases where they would help.
I live in a hilly city (Zurich) and there are a few weeks per year with snow and icey conditions, usually in January and February.
I have been convinced that riding with studded/spike tyres all winter long is a bad idea if most winter days are probably fine for normal tyres, so I've come to the two-bicycles solution to be able to decide on the right tyres without any time penalty for switching wheels.
Since my commuting bicycle is a Brompton with six gears and dynamo hub, switching wheels in the morning depending on the weather isn't an option anyways.
When I fetched my winter Brompton from the mechanic and did a test drive with the studded/spike tyres, I noticed that they're really loud and that indeed will only ride that bicycle when necessary.
I don't care about rolling resistance or possible speed. My daily commute is a inner-city ride of about 5km.


So far the winter was really dry and there was no need for studded/spike tyres.

But when should I switch to my winter bicycle with studs/spikes? My current idea is the following:


When there's snow or snow mud on the streets. (Deep snow is seldom and usually gets cleaned up rather fast, but there's often still a thin layer of slippery snow mud on the streets, especially if the city runs out of gritting/thawing salt.)
When the previous days were rainy and the weather forecast talks about freezing temperatures. (→ chance of water freezing over)
When a sunny day follows cold and snow days to avoid the case where I had my accident two years ago.  (→ chance of water freezing over)


Are there any other conditions on which I should use studded/spike tyres? Or am I overcautious due already having had an accident on unexpected ice? E.g. many years ago I easily survived this kind of weather with normal Schwalbe MarathonPlus tyres.
I've got the same tyres in 700C, but only run one bike (in this city). I put them on a couple of weeks ago and they'll come off when the weather warms up in spring. There's no harm in using them all winter. In fact on a thick layer of wet leaves they're better than my marathon plus. They are noisy, but you get used to it and it doesn't down out the traffic. 

One of the worst times round here for sheet ice is frosty Monday mornings, which often follow a sunny Sunday afternoon when people wash their cars. The residential road that doesn't drain very well is one of the few that isn't gritted on my route. 

You're in a slightly different position to me of course. I suggest a simple rule: if it was forecast to be below about 3C at any point overnight, use winter tyres. If there's visible frost on the ground, use winter tyres. If you're planning on coming back late and the forecast is for less than about 3C, use winter tyres.