Depends on the issuer. On contactless cards issued in North America,
I've never been asked for a PIN or signature on charges under $25,
which even at current prices should be a rather lengthy bus trip.

Do you use the card exclusively for contactless charges under $25?

No, I use my contactless Amex to buy most of my groceries. I tap it
on the reader at the till, then if the total is low enough ($25 at the
smaller local market, $50 at the bigger chain store) they just hand me
the receipt and I'm done. Above that, since this is the US, they ask
me to sign using the stylus on the pressure sensitive screen, at which
point I write something like NOT ME or FLUFFY and we're done.

My Canadian mastercard has both a normal chip and a contactless one.
I haven't used it enough in Canada, where they have chip+pin, to
figure out what the rules for contactless transactions are.

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