To install this mod you need Map Editor 1.1 by Guadmaz, or newer version:
https://www.gta5-mods.com/scripts/map-editor

For drag race actually to work, i used Impromptu races mod by Eddlm:
https://www.gta5-mods.com/scripts/impromptu-races

To install the mod, you must have Map Editor installed, then, 
you put the DragRace.xml (and LongTrack.xml, since update) file in your GTA V directory, folder where your gtav.exe is
When you are actually in the game, open Map Editor with F7 and choose 
Load Map, then type - DragRace - or -LongTrack- (with no - ), you should then get the loading screen and WAIT,
it may take it a minute to load all the components, after that, you are good to go and to use your new drag race track 
IF GAME CRASHES INSTALL LATEST SCRIPTHOOKV, AND SCRIPTHOOK.NET

Update 1.5
- Added a longer track (about 1/6 mile long) you can activate by typing LongTrack in the Load Map bar 
(it has around 800 props so it may cause fps drop if you have low spec pc, also rockstar editor might not work well with it)

-Improved a lot of stuff, now at the start nothing separates 2 lanes as it was problem to place AI in the correct position, 
just go to the line and race

-Added animations to almost all peds (some anim. wont spawn every time, nothing i can do)