Not a lot of people realize that virtually all AWD cars and SUVs are actually just FRONT Wheel Drive. The "all wheel" only kicks in when the on-board computer detects slippage. At which point your ESC might also kick in. There is almost always an AWD Lock button, along with an ESC defeat button, which allows you to turn on the drivetrain to all four wheels, while telling the onboard computer to stop trying to compensate for the slippery road (the ESC can be a bitch, take over your throttle and brakes if it thinks you're slipping -- in heavy snow, you want to have control yourself, not rely on a computer chip which can't tell there is a hairpin turn ahead). You have to keep your speed lower, but you probably are anyway if the conditions are that bad.
Also, I don't think AS tires are considered the same as M+S tires. M+S have the yellow and red dot on the sidewhile, and CalTrans considers them okay for R1 conditions. But they're not the same as AS tires, which I believe only have a yellow dot on the sidewall. It's spelled out in the CalTrans regulations.
I have AS tires on my AWD SUV, which is sufficient for Vermont conditions, but I'd never consider driving through the pass to Kirkwood on them if it was snowing hard. The M+S tires have a much deeper, beefier tread.
not all AWD are equal...
Subaru Impreza WRX STi (2005-2008)(Edit)
Full-time all wheel drive with 35/65
(?) torque split front-to-rear under normal conditions. Driver-controllable Center Differential System (DCCD).
“Helical-type” front differential (2005-... WRX STI), varies the torque delivered to the left and right axle shafts, depending on traction and engine load. Instead of locking the output shafts so that they rotate at the same speed, this differential sends more torque to the wheel with more grip. In addition, it makes a gradual adjustment for a more fluid response.
Full-Time All Wheel Drive
This is a permanent all wheel drive or permanently engaged all wheel drive system. All wheels are powered at all times. The vehicles with full-time all wheel drive are equipped with a center differential that lets all wheels travel different distances while turning. This type of all wheel drive can be used both on and off road. In slippery conditions, the center differential can be locked, whether manually or automatically, depending on the vehicle.
When a manual center differential lock (available on off-road vehicles and some SUVs) is engaged, the transmission's behavior is similar to part-time all wheel drive, i.e. the front and rear driveshafts rotate at the same speed. The use of full-time all wheel drive with locked center differential is limited to surfaces with low traction.
In case of an automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction device is used in conjunction with the center differential. When a wheel slip occurs (one driveshaft rotates faster than the other) the device locks the center differential and the torque is transferred from the axle that slips to the other axle that has traction. As soon as the wheel slip is eliminated, the device unlocks.
Some vehicles (Land Rover Discovery II, pre-xDrive BMW X5) do not have a locking center differential, but are equipped with an electronic traction control system (known as Electronic Differential Lock - EDL) on all four wheels. This electronic system detects slipping wheels by reading ABS sensors, then it applies brakes to the slipping wheels and the torque gets transferred to the wheels that have traction. While it performs well on slippery roads, the system cannot compete with a real mechanically locking differential when driving off-road.