Keeping it as simple as possible (look them up on a dictionary for more, please):
"Go straight" ou "Go ahead" ou "Go foward".
Go straight - used for giving directions. - "Go straight on at the lights / when you come to the crossroads."
Go ahead - also used to give directions, many times one will say "go straight ahead".http://www.bbc.co.uk/worldservice/learn ... age2.shtml
It is also used in the sense of "gaining terrain" (literally or as a figure of speech, a way of speaking.), going forward etc.
Going forward - used to mean "making progress" etc, sometimes meaning that despite change of the circumstances, strategies etc, you/they/the team etc, maintain the focus, continues on to achieve the goal.
Many would think it´s a redundant statement, and under some circumstances it may be, so be aware of that.http://english.stackexchange.com/questi ... rward-mean
Ah, "go straight" (ahead)
has a certain ring of "taking a bee-line" to it. The others, don´t, in most of the cases.