Start with a circle made of quarter-circles. The ones with the bite taken out of the middle.

Raise each connecting edge by the same amount.

But wait, that's no good!

If we straightened this out, it would look like this!

We need to make it look like this.

The difference on the straight one is that the handles are at an angle, so let's try that on our curved one.

And we're done!

From here, you can make your ramp all kinds of interesting shapes by changing the shape of the connecting edges. Try moving the outer edge upward for a banked turn!