Cars produce more emissions when idling than when in motion.
This seems like a patently false statement. How could cars produce more emissions when idling than when in motion? The engine is hardly doing any work. Perhaps different kinds of emissions are produced when idling than when working the engine harder, but the total amount I would guess is smaller when idling.
The article is comparing 2 cases, each involving driving along "a section of road with 10 traffic signal systems"
1) as today, when drivers have no information, leading to "unnecessary stopping and starting" - i.e. cars accelerate to a cruising speed, reach a red light, stop, idle, wait for green, accelerate again etc
2) in future, when drivers\self-driving cars are informed when lights will change, hence avoiding stopping and starting - i.e. cars change speed so that they will cover the distance to the lights in time for the lights to be green, therefore minimising acceleration, braking and preventing idling completely
In the second case fuel consumption is reduced - hence "Cars produce more emissions when idling [case 1] than when in motion [case 2]."