I just read an article by Linda Kirkpatrick - Executive Vice President, U.S. Market Development at Mastercard - "No More Signing on the Dotted Line".
According to the article, Mastercard will no longer require signatures at checkout for any credit or debit purchases in Canada and the U.S. starting April 2018. Right now the card brands (VISA, Mastercard, Discover, American Express) have thresholds for transactions that don't require a signature, typically $50. But starting next April, Mastercard won't require a signature for any retail transactions at checkout. I imagine that the other card brands will follow suit at some point.