In late February, Rep. Doug Pickett, R-Oakley, told his colleagues at the Idaho Statehouse about a proposed legislative fix to what he described as a fatal problem on the state’s major highways: differential speeds. The bill he sponsored sought to eliminate Idaho’s bifurcated — or split — speed limits, which for over a decade have allowed cars to whiz along some rural stretches of interstate at 80 miles per hour, capping heavy trucks at 70 miles per hour.