The only thing that matters is making sure you don't get caught: to that end, you either need to make sure there aren't any witnesses around or you need to "silence" them before they report you to the others.
However, even this isn't foolproof: occasionally, the victim of your theft will send hired thugs to kill you even if there were no witnesses.1 Lady Justice may be blind, but she's got some great spidey sense.
So stealing on the main road, when people are around, is a patently bad idea. Stealing when people aren't looking or when they're asleep would work better, but it's still not a guarantee.
Note 1: The hired thugs event is random and isn't triggered by a value threshold either.