DWI Time of Driving vs. Time of Test

An essential element of the crime of DWI is that the person is intoxicated at the time of driving. However, chemical tests only reflect a person's alcohol concentration at the time of testing. A person's alcohol concentration at the time of driving may have been higher, lower or the same. In order to link the test result to alcohol concentration at the time of driving the prosecution may attempt to present expert testimony concerning alcohol concentration at the time of driving. The process the expert uses to relate the test result back to the time of driving is known as retrograde extrapolation. However, in order for the expert to offer an opinion which will be admissible at trial, the court must find that the expert's opinion will be reliable.

Factors effecting reliability include:

• 1. the length of time between the offense and the test(s) administered;
• 2. the number of tests given and the length of time between each test; and
• 3. whether, and if so, to what extent, any individual characteristics of the defendant were known to the expert. These characteristics and behaviors might include, but are not limited to:
  • 1. weight and gender
  • 2. typical drinking pattern
  • 3. tolerance for alcohol
  • 4. how much the person had to drink on the day or night in question,
  • 5. what the person drank,
  • 6. the duration of the drinking spree
  • 7. the time of the last drink, and
  • 8. how much and what the person had to eat either before, during, or after the drinking.