SAN DIEGO (CBS 8) - Whether it is alcohol or marijuana, driving under the influence is a crime, and testing drivers who are high is a lot tougher.

Law enforcement is having a difficult time proving whether or not drivers who admit they have smoked marijuana are still under the influence of the drug while behind the wheel.

Colorado - where recreational marijuana has been legal for a couple of years - is also facing the same problem as California.

The problem is that blood tests will not indicate the THC levels in a person's system. Current tests do not inform officers if someone smoked marijuana in the past five minutes or the past five days.

UC San Diego is leading the way on designing a more accurate test for driving under the influence of marijuana.

The goal is to design an iPad app for law enforcement to use in the field and help keep everyone safe.

A recent statewide poll shows 58-percent of likely voters support Proposition 64, 39-percent oppose it.