There's plenty of them
I don't see it where I live, Des Moines is the biggest town and it slows from 70 to 65 about a mile out from the beginning of the urban area, the belt way stays 65 and the leg that goes through downtown reduces to 60 then 55 where there are exits about every .5 mile, then back up to 65 when you leave downtown. This is for I-80 and I-35
I live 30 miles north of Des Moines and I-35 slows from 70 to 65 for the two interchanges we have, they only slowed it down after a year of daily accidents at the US Hwy 30 and I-35 cloverleaf.
The biggest problems we have in Iowa with Interstates are people who feel that traffic needs to yield to them as they enter the Interstate and the habitual left lane driver.