[Doady]

Grade separation of a bus or rail corridor (including light rail or commuter rail) is to allow for increased capacity of the corridor, either increased frequency or longer vehicles or both, without that service interfering too much with the regular road traffic, either auto or pedestrians or bikes.

With light rail and commuter rail, there is also the option to combine vehicles and cars into trains to increase the headway (reduce the frequency of service), and so reduce the disruption to other traffic, or the reduce the disruption other traffic causes to the transit service.

Sometimes you will find a bus or rail crossing is grade-separated because the bus and rail service is too busy, but given the low ridership of many systems, it is because the road is too busy with cars, trucks, bikes, and/or pedestrians.

Around 10-minute frequency in each direction with trains of LRVs is good capacity. Signal priority can be applied without much disruption to other traffic. Each LRV is probably around 30m (100ft) long, so each train 60m long. But if ridership increases too much, maybe higher frequencies and/or 90m or 120m trains would be needed, and so the disruption to other traffic could be too much, and that is when grade separation is needed.

The ability to combine vehicles into trains is the main advantage of light rail vs. bus rapid transit. Articulated buses are usually only 18m (60ft) max, only 50% longer than regular buses. The capacity of BRT is less than 1/3 of light rail with 2-car trains for a given frequency. So when we talk about BRT vs. LRT, it's really a question of the capacity needed. And of course, capacity is exactly what defines heavy rail as well. High frequencies and long trains? You need grade separation for that, and that is what makes a rail line heavy rather than light, or full rapid transit. Light rail and bus rapid transit therefore can be considered as semi-rapid transit, having some features of rapid transit (all-door boarding, limited stops, exclusive ROW, grade-separation) but not all because the ridership doesn't require it, the extra capacity is not needed yet.

Capacity is why Ottawa needed to replace its grade-separated BRT with rail. The problem wasn't lack of ridership, it was too much ridership. Ottawa is one of the leaders in transit ridership, and the articulated buses could no longer handle it. The difficulty of articulated buses to operate in snow only made matters worse. Similar problem could be seen in Mississauga, with buses coming by every 3 minutes in each direction in one corridor, but buses still too full and sometimes not being able to stop and having to leave the riders stranded and continue waiting. The fact that articulated buses had to be left in the garage during periods of heavy snowfall. So the service needed to be replaced with LRT, which is now under construction.

The question with Los Angeles or any US system is whether their problem is too much ridership, or is it not enough ridership? Which bus corridors are becoming so crowded that they need to replaced with light rail? Which light rail corridors are becoming so crowded that they need grade separation to allow for longer and more frequent trains? That is all any city needs to think about when considering building new rail or upgrading existing rail. Canadian systems are building new rail but not because of lack of ridership, and maybe the USA needs to taking that same approach more.