making efficient map layers

[Jeff Portwine]

Last week I ran into a problem where I created a tiled layer of tiger line roads, and when I tried to view the whole area of my map (which is basically the NE portion of the US from pennsylvania up to Maine) it caused the server to use up all of its memory and start shutting down processes.    It was pointed out to me, that even though I was only displaying one particular road type which by itself wouldn't be a lot of data, mapserver still had to inspect every road in the shapefile to match it to my expression and therefore was very inefficient.

So, if I'm understanding correctly, a better way to do it would be to break the shapefiles down into individual road types and have a separate layer for each type... for example I could have an interstate shapefile, a state highway shapefile, a major roads shapefile, and a minor roads shapefile and make a separate layer for each of them.  

How can I pre-process those shapefiles to separate them into their components?   I don't necessarily have to tile them, I was only doing that so that I could combine all of the shapefiles into one (the tiger shapefiles are per-county so there are a ton of them).   If there is another way to combine those into one I could do that as well.    I just don't know how to break the tiger road shapefiles into individual road types.   

Thanks for any suggestions and info,
Jeff
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