From the study: "A road diet involves narrowing or eliminating travel lanes on a roadway to make more room for pedestrians and bicyclists. (They) are often conversions of four-lane, undivided roads into three lanes-two through lanes plus a center turn lane."
The study was conducted in California, Washington and Iowa:
"The Iowa data indicate a 47 percent reduction in total crashes while the HSIS (California and Washington) data indicate a 19 percent decrease-a substantial difference."
It's clear from the results that road diets do reduce crashes - the only issue appears to be just how great a reduction.
Thanks to Transportation Research E-Newsletter