I support this. Where I live and commute ther are lots of tunnel systems, and they are long. I have submitted a suggested "Tunnel" road type in the Cartouche map editor.

http://waze.uservoice.com/forums/59225-cartouche-map-editor-and-waze-com-website/suggestions/1143093-handling-tunnels

oh and please snap current location to tunnels only when there is no gps signal (or something) - last time i had waze wanting me to jump of a cliff that has tunnel unerneath.

Yep, would be a great idea! One would need to have a specific part of the street reported as tunnel, otherwise waze cannot differentiate between general loss of gps signal and a tunnel where it is supposed to keep the calculated average speed...

You could use the compass for this partly at least...

perhaps use phone's accelerometer to estimate change in movement?

I think tomtom and others have been doing this for a while, and it's pretty simple!
you have the speed at the entrance just keep the icon moving at the same speed when you loose the signal entering the tunnel and correct any eventual errors once you exit and get contact again!

Or you can calculate the general speed by taking the distance / time in tunnel and set the "car" to move in that pace and then correct it once you enter!

Sure, it won't be a 100% match always, but it's one way to do it!

How exactly are you planning to do this without GPS? If there is no GPS available (such as in tunnels), you need to depend on cellphone towers, but these are inaccurate in displaying the exact location (and several tunnels do not have towers, but have the signals coming from outside (or beginning/end of) the tunnel).