Matthew J. Ruff DUI/DWI Law Blog on Lawyers.com

Drunk Driving in Los Angeles is treated very seriously by the District Attorney and City Attorney of L.A..  Because of the popularity of tourism in Los Angeles, the police want to send a message that it is safe to visit the city and that tourists can feel comfortable in knowing that they can walk the streets near the popular LA attractions, the Los Angeles worldwide sites and other popular destinations and not be hit by a drunk driver.   

In my experience as a Los Angeles DUI Lawyer, the police will look for certain tell tale signs in order to justify a vehicle stop and detention for suspected DUI.  Research by the National Highway Transportation Safety Administration has identified the following indicators of drunk driving, in descending order of probability that the driver is intoxicated, suggesting a high probability that a driver exhibiting the behavior is intoxicated:
Turning the car with a wide radius
Straddling the center or a lane line or marker (sometimes called lane straddling)
Appearing to be drunk or visibly intoxicated (e.g. gripping the steering wheel tightly, driving with one's face close to the windshield, slouching in the seat, drinking in the vehicle, or staring straight ahead with eyes fixed)

Almost striking a fixed roadside object or vehicle
Weaving within or outside a lane of travel

Driving the vehicle somewhere other than the designated roadway (e.g. on the
shoulder or straight through a turn-only lane)
Swerving in a sudden fashion
Driving at a Slow speed (10 m.p.h. or more below the speed limit)
Stopping the car in the  lane without cause
Following another vehicle too closely, improper or unsafe distance
Drifting in and out of the lane
Tires traveling on center or lane marker
Braking suddenly or erratically
Driving into opposing or crossing traffic
Signaling a turn inconsistent with driving actions
Stopping inappropriately other than in lane
Slow response to green lights or other traffic signals
Turning abruptly or illegally, in violation of traffic laws
Accelerating or decelerating rapidly
Headlights off at night, (for example when leaving a bar)
The scientific concensus is that if more than one indicator is observed, it is even more likely that the suspected drunk driver is intoxicated .  The most important observation is that speeding is not an indicator of DUI; because of quicker judgment and reflexes, in some circumstances it may indicate sobriety. Once a stop is made, the DUI police officer should look for the following behaviors by
the driver, as NHTSA has found them to be excellent predictors of DUI:
Difficulty with vehicle controls;
Difficulty exiting vehicle;
Fumbling with a drivers license or registration;
Repeating questions or comments to the officer;
Swaying, unsteadiness or balance problems while speaking to the police officer;
Leaning on the vehicle (or other object);
Slurred, thick or drunken speech;
Slowness in responding to questions, or asking officer to repeat questions;
Providing incorrect information or changing answers;
Odor of an alcoholic beverage.