Gate Skip is a WTFTPAW which allows you to go beyond the train station without needing Poison Jam to break down the gate. This eliminates the need to race Poison Jam in Chuo, saving several minutes.
By boost jumping through the wall texture at a certain angle and then curving right, you can land on the edge of the ground that extends out-of-bounds. Afterwards, you simply death warp at a certain location and the game will spawn you beyond the gate, allowing you to complete the level.
The gate will remain closed until you complete the graffiti mission. If for some reason you want to return to the beginning of the level without completing the graffiti, you must do Reverse Gate Skip (see below).
If ever you wish to return to the beginning of RDH without completing the graffiti mission, you must do this OoB/death warp. You must boost far enough and with a good enough angle that you fall through the water and hit the void plane; this is what takes you back to the beginning. If you get a drowning animation, you likely did not boost far enough and will spawn at the wrong location.
Since this OoB takes so long and is a bit tricky, it's always faster in a speedrun to just complete the graffiti mission and climb the level twice than to do this.
You can barely complete the L just beyond the train station without skating into the cop trigger. However, you will be unable to continue as the trigger completely cuts you off from the rest of the level. Due to this, we skip this graffiti after Gate Skip so that it's the last graffiti we complete. Saves about 15 seconds.
You can barely complete the upper level M without skating into the cop trigger. You must jump across to the other side of the trigger, and carefully spray the M in two parts, minding your speed so you don't skate into the trigger. This skips the biggest cop fight in the game and saves about a minute.
On the upper level, there is one blue can that is just inside the wall in the corner. You must skate into the wall at a somewhat precise angle to collect it.
The ZulBro is a famous sequence of frameboosts and is the fastest way to get from the RDH entrance to the Sewers. You start by boost shuffling into the halfpipe rail and frameboosting there, then two more frameboosts around the curve of the sewer pipe into the load zone.
By doing Gate Skip and activating the lower cop fight from the side opposite the train station, you will be on the wrong side of the electric fence. You can then travel by way of the sewer pipe and discover that the load triggers for the level are now disabled.
An OoB you can do by boosting into a WTFTPAW to get into the buildings near the entrance.
From the pool area, you can boost jump off the return rail into the buildings left of the train station to get beyond the sewer gate. The halfpipe is perfectly functional, but the water is fake.
By jumping at the "front" side of the trailer at a certain angle, the player can be warped across the map, similar to the Hoop OOB in the Garage.
You can boost into the entrance pipe and a certain angle, and your speed will clip you through it.
A tiny OoB you can do at the 2nd level of RDH.
By boosting into the back of one of the last billboards in the level, it can push you though the wall and out of bounds. This will make you fall to a much lower level.