Tunnel extrapolation is an extrapolation function which uses the last available speed data to extrapolate the position of your vehicle inside a tunnel.
Add tunnel_extrapolation="12" to <tracking ... >. You will need to play around with this value if you find that the cursor lags or runs ahead of your actual position. 12 stands for 1.2s extrapolation per gps rate.
Note that this function will only work if the particular section of highway has the tunnel=yes key/value pair in OSM (and if it hasn't, go ahead and add it!). To process your own map, see OpenStreetMap#From_.bz2