Last week's answer

Question: Diverging Diamond Interchanges (DDIs) are rapidly replacing traditional diamond interchanges across North America because they drastically reduce T-bone crashes and keep traffic flowing. When drivers enter a DDI, traffic temporarily switches to the left side of the road before crossing back over to the right. What primary geometric engineering goal does this temporary crossover achieve?

Answer: It allows drivers turning left onto the highway on-ramps to do so freely without ever crossing in front of oncoming traffic or waiting at a second traffic sign

Crossovers gently transition traffic from the right side of the road and then back again. As left hand turns are completed from the left side of the road, they do not have to cross traffic. 

Sources: USDOT