The way that I would try to do this is to add a field to your roads and update it with a count of vertices.

 1. Add a Long Integer field
 2. Calculate that field using the Python parser to be equal to `!shape.pointcount!`,

You can then use the vertex count field as, or to contribute to, your impedance field.

You'll need to think about whether an almost straight road with lots of vertices should be treated as "straight" for your purposes, and maybe also about how you will deal with roads that consist of many lines.