That is primarily because the prevailing wage is calculated by agencies like erieri and Cal EDD (in the case of California) surveys which are usually nine month

That is primarily because the prevailing wage is calculated by agencies like erieri and Cal EDD (in the case of California) surveys which are usually nine months to a year old. So ymmv. If the survey was taken during a boom year and is used in a bust year, the prevailing wages are usually higher than market. The reverse is also true. There is also a selection bias in the data as they only have data from organizations which are willing to share this. There is no mandated requirement for this. Another thing to keep in mind is that (in theory) prevailing wage data reflects market wages (i.e. all types of engineers) not just those on a visa. Whether or not this is actually what happens is something I have not idea about. So in theory the LCA data should be a good datum to start your salary negotiations on. However it should be tempered by current market conditions, location and most importantly your own position in the negotiations. Its always easier to negotiate from a position of strength. When you are looking to provide for your family, insurance, rent/mortgage etc and are unemployed, negotiating may or may not work depending on how desperate you are.