Your 'Huh?' was less than explicit

Because I felt completely at sea. I understood each word individually but none made sense up next to each other.

Your explanation was clarity itself. What you call a Slip Road we would call an On Ramp or an Off Ramp. That which you describe as a One Way System would either be a divided highway or, we have sometimes in cities or towns cases where a major road going through the town will have pairs of one way stretches. I would have thought that changing sides of the road wouldn't cause any problems. We have a One Way System in Ithaca on the main road through town and I couldn't imagine why anything about it, beyond some things that would require some lane repainting, would seem illogical if we were driving on the other side of the road. Similarly for the on/off ramps. It would have seemed that any speeding up in one direction would be mirrored by slowing down in the other.

But apparently I'm wrong. If you've experienced this in real life, I'll not doubt you.