The role of concentrated fencing around urban areas in particular is to prevent immigrants from blending immediately into a town population, and in that sense, it is effective.1Even if you divert some illegal immigrant traffic elsewhere along the border, fencing still reduces overall rates of crossing by forcing those who would cross to go through more dangerous and barren territory; this is a significant deterrent. 2 Additionally, you can step up border patrols in the areas that do not cover the fence to catch drug smugglers and other illegal border crossers.3 This reduces the numbers of border patrol agents necessary to create an effective net to catch would-be illegal immigrants, and consequently reduces the long-term costs of border protection measures. The fence is meant to be merely a tool in the tool box, not a comprehensive solution to the problem of illegal immigration.4  1Associated Press. "U.S.-Mexico border fence almost complete."  2Wood, Daniel. "Where U.S.-Mexico border fence is tall, border crossings fall."  3Hendricks, Tyche. "Border security or boondoggle?"  4Archibold, Randal and Preston, Julia. "Homeland Security Stands by Its Fence."