Yes.
In Washington DC commuting traffic, splitting lanes is illegal but not unusual. The system of DC streets was designed by Charles L'Enfant in 1791 for horse-drawn carriages, so they are narrow. We now have delivery trucks & busses regularly block curb lanes and the DDOT answer to every problem is to shut down the few remaining lanes. We get VIP motorcades for every minor head of State bringing their family for a shopping trip to the US. If a President decides to go to a restaurant or drive to Andrews AFB due to weather, streets are typically blocked for roughly 15 minutes before his roughly 13 vehicles pass by and yes this happens at rush hour. Yesterday the cabbies shut down part of the National Mall area to protest Uber and the other ride sharing services.
The District of Columbia really does gridlock every rush hour, morning and evening.
Outside of the US & in California it's normal, legal, and expected. You filter up to every red traffic light and people wonder why you are taking up the space of a car when you don't. I used to live in Cali and still ride overseas regularly, so have about 30 years experience at it. (Somebody with 40 or 50 years can chime in next.) Unplated scooters (less than 50cc engine size) can do it legally. Splitting (aka - lane sharing) can be done safely and there is not a technical reason to take up most of the space of yet one more car.
It can also be done dangerously, just like almost every other activity.
- Ahem - If I were to do said illegal act, it would hypothetically be at a walking pace when traffic is jammed, watching the closest cars for somebody to do something bad, and watching the cars farther ahead for police vehicles. When the white line gets too narrow, I don't push, I stop and wait.
As opposed to "reckless-driving/pull-license/tow-the-bike offense" I've received 3 warnings over the years, and have no other violations. If/when I do get a ticket, I'm sure that by the local law I will have earned it and won't contest it.