1800 cars/h means a continuous uninterrupted stream of traffic, which is a reasonable assumption for motorways and other grade-separated roads, but less so for

1800 cars/h means a continuous uninterrupted stream of traffic, which is a reasonable assumption for motorways and other grade-separated roads, but less so for an urban road, where you will have frequent other traffic wanting to cross at-grade. So for urban roads it's typically intersections that are the limiting factor for capacity, not the stretches of road inbetween.