Why do US states generally set the legal BAC for drunk driving at 0.08 or 0.10?

The illegal per se limit was set at 0.10% for many years. NHTSA mandated the lower limit after research indicated that virtually everyone suffered impairment at that level. As said, any state that ignores the NHTSA guidelines risks loss of highway safety funds from the federal government, and no state is flush enough to do that. In the 1980s, the American Medical Association advocated setting the limit at 0.05%, saying there was impairment at that level. The "illegal per se" clause of most DUI laws makes it unlawful to drive or be in actual physical control of a vehicle with 0.08% or more blood alcohol content (BAC), no matter how experienced a drinker one happens to be. This provision is to counter the (fallacious) argument that some people may be impaired at that level, while others are not. This doesn't mean that you're necessarily off the hook if your BAC is below 0.,08%. In most states, one can still be convicted of DUI if impairment can be shown, even if the BAC is below 0.08%. Some states have a provision that a BAC below 0.05% is evidence one is not impaired by alcohol. .

Supplementary to the other answers, you can find relevant discussion in these 2001 NHTSA technical reports: http://www.nhtsa.gov/people/injury/research/pub/alcohol-laws/08History/index.htm#CONTENTS and http://www.nhtsa.gov/people/injury/research/alcoholhighway/index.htm#Contents or this tl;dr version: https://docs.google.com/a/caa.columbia.edu/viewer?a=v&q=cache%3AOQ7lG43fIpUJ%3Awww.nhtsa.gov%2Fpeople%2Finjury%2Fnew-fact-sheet03%2Ffact-sheets04%2FLaws-08BAC.pdf+legislative+history+tea-21+bac&hl=en&gl=us&pid=bl&srcid=ADGEESgNNsexc_MooJvvPryC51wX4WIGz4KseYax8qr6oDPhubc7754qKua9S9I1-RpS3T3er-fVpDUiL-EG-M2bjpZ-P01ANDX8ia2jVTRDLKkLT6CyvmeYY_W_pslxXLH20upiQzzU&sig=AHIEtbQfdjAV8d3WldFcpWtTzU510bGqtw&pli=1 See also the 2003 Notice of Proposed Rulemaking requesting public comments on the proposal (https://www.federalregister.gov/articles/2003/02/06/03-2790/operation-of-motor-vehicles-by-intoxicated-persons), and the final Federal Rule implementing the program (https://www.federalregister.gov/articles/2003/08/22/03-21492/operation-of-motor-vehicles-by-intoxicated-persons).