Police are puzzled to explain a steady, four-year decline in people arrested on suspicion of drunken driving in McHenry County.

From Crystal Lake to Bull Valley, officials point to economic factors that affect people’s willingness to dine and drink out, and constraints on their own patrol resources as possible reasons. They also are seeing more residents use designated drivers, the result of public awareness campaigns about the hazards of driving under the influence and stiff DUI penalties.

The trend is clear. DUI arrests in the county and across the state are steadily declining. From 2009 to 2012, the county saw DUI arrests decline by 207, data from the McHenry County Circuit Clerk’s Office show.