Unfortunately, wizards released nothing for modern that has included a comprehensive list of encounter tables, Critical locations provides lists of the types of people you are likely to see in the various buildings listed in the book. However, here is perhaps the best solution I can come up with, take the basic DM's Guide encounter tables (Forest, Plains, ect) from here you can substitute the creatures that do not exist in your campaign, with the mutants, bandits or whatever you feel works best. Urban encounters I'm less sure about.