Naturally, it can be a bit hard to track how many drivers do not have insurance, as they don't report driving illegally. However, statistics from 2009 can give a ballpark figure that most people assume can be extrapolated out to today's highways at roughly the same rate. The actually study was done in 2011, looking at the full stats from two years before, and it was carried out by the Insurance Research Council.
What they found was that about 14 percent of the drivers in the United States do not have insurance. That means that roughly one out of each seven drivers that you pass on your way to work probably does not have coverage.
While this is a national total, it does matter where you live. Colorado isn't the worst in the country; that distinction goes to Mississippi, where about 28 percent of the drivers did not have coverage -- or more than one out of every four.
States on the East Coast tend to do a bit better, statistically speaking. Four of the states with the lowest numbers of uninsured drivers were Maine, New York, Massachusetts and Pennsylvania.
As you can see, the odds are that you're going to run into a driver with insurance in any accident, no matter where you are. However, noting where things are the worst and the fact that all states have some uninsured drivers is important, as you need to know what to do after you are involved in an accident with one of these drivers. This is why it's so important to ask the other driver to exchange insurance information with you immediately after a crash.
Source: How Stuff Works Money, "What happens if you're in an accident with an uninsured driver?," Linda C. Brinson, accessed April. 28, 2015