That is a good question to ask if you are planning a trip. A good source is the NTSB website. They have tables full of Statistics so I will try to summarize it. It is also good info on crash investigations and has reported findings that make for some good information—although it might scare some people away from flying.

Scheduled US Airlines Stats for 2006
25 Accidents with 2 fatal accidents out of 11,200,000 flights.
Total 50 fatalies
Accident rate per 100,000 is 0.223 or 0.018 for those with fatalies.

General Aviation in US for 2006
1,515 accidents with 303 fatal ones out of 22,800,000 flight hours.
No rate as not all private flights are recorded.
This data is under Aviation menu of NTSB site. I searched for similiar statistics for accidents on the highway but did not see any.
Reference: www.ntsb.gov/aviation/aviation.htm

Here’s an interesting statistic. If you took a flight every day, from any random airport to any other airport, it would be 6,000 years before you are statistically likely to be involved in an air accident, and the chances are that you would servive it.