It is also for firetrucks and ambulances so they do not have to stop <| sep |> Yes , I agree . There are persistent irrationalities . However , there ' s a wide chasm between being able to SHOW irrational prices such as trending following good news , and overreaction and reversal following bad news , and being able to EXPLOIT them . And there ' s quite some strategies that have shown to provide abnormal returns when accounting for transaction costs . . any evidence of that ? Best I ' ve found in my research is a few percent over individual securities ' return WITHOUT accounting for transaction costs in a matter of months e . g . Thaleris Barber , or lowlatency hifreq mean reversal correlation strategies using e . g . Epps effect across markets . However , the latter is EXTREMELY susceptible to actual price movement since the alpha is so slim , a single rare directional move wipes out all accrued returns and then some . This is one reason why the number of quant funds has sharply fallen over the last few years . Unless you ' re truly the first aka the fastest the remaining alpha does not outweigh the risk .