Sobriety checkpoints have proven to have the greatest deterrent value of all methods used to prevent DUI. The Centers for Disease Control and Prevention found that sobriety checkpoints consistently reduced alcohol-related crashes by about 20 percent. Cost and the large number of officers required were among the reasons most often given for not conducting more checkpoints. A study in two West Virginia counties found, however, that small rural communities could safely and effectively conduct weekly checkpoints with only three to five officers. The study found that low-staffing checkpoints at a cost of $350-$400 per site could result in large reductions in the number of drivers with higher blood alcohol concentrations. Another strategy for countering DUI has been used by the Montgomery County Department of Police (Maryland). Since 2001, this department has conducted a civilian volunteer program called "Extra Eyes." Volunteers, typically citizens' academy graduates, use their own vehicles to identify DUI violators. Working in pairs, when the volunteers see a violation, they relay the information to nearby officers. An officer then monitors the drivers to determine whether probable cause exists for a vehicle stop. Volunteers are trained to identify signs of DUI. MADD and the IACP have joined together to provide services and material for law enforcement agencies and communities to use in developing their own strategies and campaigns for countering DUI.