Police are fighting a nightly battle to keep dangerous drivers off Long Island's roadways.

Sometimes, they catch drivers who have had too much to drink, but many drivers are found to be on drugs. Officers begin learning to spot the difference in the basic training they receive at the police academy.

Police recruits get to see the effects of alcohol firsthand. Volunteers come to the academy and are monitored closely for several hours as they have drink after drink. The volunteers are then given tests by the recruits.

Some police officers go on to become drug recognition experts. These officers are highly trained to use their powers of observation to determine what kind of drug a person has used. They can be called to the scene of a traffic stop, or to conduct further testing at police precincts. Suffolk County has 12 DRE officers, while Nassau has five of its own.

Unlike training for alcohol detection, drug recognition experts cannot monitor and test volunteers who show up and take illegal drugs. Instead, officers have previously trained in jails where they monitor people who were already under the influence of drugs.

Last year, Suffolk police arrested about 2,600 drivers who were under the influence, while Nassau police arrested about 2,000.