When someone pays you 20 bucks do you check and re-check the banknote, call the federal reserve and ask what features to look for and what they should look like on a genuine banknote? Same applies for Bitcoin: to carry a massive 51% double spend attack you need so much computational power, so much money that doing it for a taxi ride is ridiculous (and not worthwhile). P.S. did you know that DIY checking a banknote (as most people do) is pointless if you do not know exactly what a genuine banknote should look like (features, contrast, texture, ink, holograms, watermarks etc..)?