I'd have to see some evidence of this; a scientific study. For the sake of argument, I'll assume you're right, they cross perpendicular. Maybe it isn't what they see, but what they feel on the edge of the road, both on their feet and drag on their shell, that orientates them. It could be an instinctual orientation maneuver in order to overcome obstacles in nature.

how is it that terrapins always cross the road so perpendicular to the direction of traffic? I can't recall ever seeing one doing otherwise. maybe they can see it but I didn't think their Vision was that good. especially considering their vantage point. a mystery of the terrapin to me.