Risk and speed limits are already mathematical. See the 90th percentile rule. If self driving cars makes obvious the disparity between the letter of the law and actual practice then the law will get changed.
Pretty much the only stakeholders that want the current mismatch between 90th percentile speed and speed limit are the cops who want a pretext to go fishing and the states that want revenue.
Self driving taxis, delivery vehicles, etc that are doing (for example) 55mph "because the law" when the road should really be an 80 is a terrible user experience, especially on roads with mixed human/ai traffic. Insurance, self driving fleet owners, car companies, etc all have a huge incentive for self driving cars to be able to get from A to B just as fast as a human car. Nobody will take your self driving taxi if a human taxi is faster and comparably priced.
We'll see a higher speed limits because the automakers, fleet operators, etc will lean on the states and if that doesn't work they'll lean on the feds (nhtsa, dot and similar) to lean on the states to set limits that are actually consistent with the 90% rule.