Freeway tunnels

One thing that has impressed me about Brussels is that although it is a relatively large city, the traffic is not a significant problem.  Of course, I realize that we are here in August, and many Belgians may be on holiday, but I've been surprised by the number of parking spaces available on our street at any given time.  When we went to Bruges yesterday I was anticipating that it could take quite a while to get out of the city.  In fact we were able to get out relatively quickly.  Brussels has a series of underground tunnels that allow freeway traffic to flow fairly quickly through the center of the city.  The tunnels run underneath streets and parks (I'm not sure if there are any buildings on top of the tunnels).  Each tunnel is one or two km long, and then surfaces for a short distance, at which point traffic can transition onto above ground streets or move from those surface streets onto the freeway.  The freeway quickly goes back underground for a km or two and then comes back up for more exits and entries.  It seems like a very efficient system, and the traffic seems to move at an efficient speed while still allowing for a lot of pedestrian and bicycle traffic on the surface.  Once you are in the tunnel system you can very easily emerge onto the surface relatively close to your final destination.  Even though we drove in the system during rush hour, the traffic was traveling at full speed or close to it.  There was some construction at one end of the first tunnel, which slowed entry into the system from the above ground freeway system, but once we were in we moved surprisingly quickly.