Peak bandwidth of roads happens at surprisingly low speeds, you wanna maximise bandwidth set the speed limit to about 20km/h [1]. Anything higher and road capacity starts reducing.
So reducing the speed limit down to 60/h actually increases the capacity of the road, and reduces the likelihood of rolling traffic jams occurring or persisting.
This is a question I have wondered all the time (predominantly while stuck in traffic). Frequently wondered if, during rush hour, posted speed limits should be dropped by X to actually increased throughput.
Having not yet read the paper, I am curious if this was factoring in just typical average speed or if lower speeds are also going to have fewer/less severe accidents (accidents having an outsized impact on my road delays).
So reducing the speed limit down to 60/h actually increases the capacity of the road, and reduces the likelihood of rolling traffic jams occurring or persisting.
[1] https://www.researchgate.net/figure/Road-capacity-change-wit...