Mann,

A great reference on bridge widening is Xanthakos'"Bridge Strengthening and Rehabilitation".  What makes this text so valuable is many of the examples and ideas are actual case studies from many DOT like NY, IL, CA, WI, WA and so on.  There is a good mix of large state work with some of the smaller states.

Another good reference is Parsons Brinckerhoff's bridge rehabilitation book ( I don't remember the exact title).  This book doesn't have the case studies and input from the DOTs though.  Most DOTs are conservative and like tried and true methods.

Also, if you like leave me an address to reach you and I can help out as well.  I have many years of bridge experience (new, rehab, etc.) all over the midwest.

A good source of information is the Caltrans (Calif DOT)Bridge design Manuels.