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You never really notice until you're pretty inebriated. One time I completely forgot I had two beers while working in the house on Sunday, and I left with the car to wash it: I'm very strict with my wheel skills, and extremely reliant on muscle memory and reflexes, and that was the most horrible 500m drive ever. Nothing matched my muscle memory. I was in f***ing driving school all over again, needing to baby sit every damn decision I made.
I'm using this Android game / glorified whack-a-mole simulator to objectively gauge my reflexes and determine when I'm sober enough to drive.
With 2 thumbs on the normal test I can finish in around 5.5-6.5 seconds when sober, and that can go up to 10 on even one beer.
This other test is slightly easier to compare: ~0.25 sec when sober, up 0.75 after ONE beer.
I don't remember the exact numbers for the first test, sorry: I don't drink and DD that often.