My gut reaction is that 6' seems quite tall. It might seem like the wall of a big old-timey fort, but not in the good way.
Maybe grab a yardstick and take it out there and get away from it and look back to see how tall 3' is. When you think about it, even guard rails aren't even 3' tall, and they almost always work.
My other thought is that it might look best if you ramp up the height a little, to make it kind of trapezoidal, instead of a big square or rectangular behemoth.
Of course, that might give those cars an unwanted DUKES OF HAZZARD opportunity if they hit your barrier just right!
