[Mapcss] relative width for overlays

[Sebastian Klein]

Hi,

I would like to create transparent overlay for bridges and tunnels that 
is 4 pixels wider than the main line style. How can this be done without 
repeating all highway types?

way[bridge=yes]::bridge-over {
     color: #000080;
     opacity: 0.5;
     z-index: 0.01;
     width: <?????>;
}

Sebastian