Racing Groove and Holding Your Line
The images below have been provided as examples of the racing groove, the high and low lines, and drivers holding their lines when in traffic. Please note the dark area of the track, AKA the groove in the first screenshot, below.
As you can see, the groove is wide enough to allow two drivers to run side-by-side at speed without incident, as long as each of them holds their line.
In the examples below, the driver on the outside has actually given more room than necessary, and could be running a car's width lower. Drivers are responsible for using their own discretion and awareness of the situation in choosing their lines and giving room for other drivers.