Curvature is certainly one, remember that different cars handle turns differently and the bank of the road can allow for faster turns than a non-banked road. Highways have a significant bank, but that depends on region because freezing conditions limit the ability to bank a highway (imagine stopped traffic on an icy road, you can't have them sliding toward the median!).
Another important factor is distance-to-stop. In some cases there is a median blocking the view around a corner. All else being equal the turn should be fine at 65mph but because of the lack of sight distance they have to lower it to 55mph.
Now the actual speed posted on the road doesn't always exactly match the engineering designs. For example, the PA turnpike recently increased the speed limit for most of the highway from 65mph to 75mph. Over the last 50 years they've done a lot of incremental enhancements on flattening turns, adding a 3rd lane, etc. but my dad has told me that the increase to 75 was dubious due to turning radius still being unsafe at 80mph over certain sections.