In all the conversations about Gerrans, Cancellara, and Nibali's breakaway at Milan-Sanremo (centered on whether Gerrans' performance was "dishonorable," whether Cancellara as the strongest rider "deserved to win," and so forth), it's easy to lose sight of the speed involved. So, with a calculator in hand, let's go to the videotape.
THE POGGIO: A comparison of the Poggio map with a route tracked on Ride With GPS shows that yes, as reported, the climb is 3.7k, rising 570 feet. The lead trio climb in 6:35, from the time the peloton's first rider enters the road up the Poggio to the time that the trio makes the distinctive left turn to the descent. This is an average climbing speed of 21mph, or nearly 34kph. It's reasonable to say that their top speeds while climbing were significantly higher - when Nibali and Gerrans blasted away from the pack, and when Cancellara strung things out in his bridge to them such that the elastic snapped.
THE FINAL KILOMETER: Later, we see the leading trio go under the red kite, signaling 1km to go, at 12:37. At 12:47 they pass a crosswalk, and you can see the first chase group hit the same crosswalk 5" behind. They cross the finish line at 13:42, having crossed the final kilometer in 1:05 , or an average speed of 34.5mph/55 kph. At the end of 7 hours of racing! The first chase group hit the line :02 behind. Their last-kilometer speed was 36mph/58kph.
If Cancellara, Nibali, and Gerrans let their average speed drop just 1 mph in that last kilometer, Peter Sagan might be the winner of Milan-San Remo by a bike throw.