The Warriors uses the renderware engine.
3 titles use the EXACT renderware engine and were all developed "Back to Back":
- GTA SA
- Bully
- The Warriors
R* North handled GTA SA, Rockstar Vancouver handled Bully and R* Toronto handled The Warriors. Rockstar has been known to "Share" code with other offices, so its very likely vancouver at one point in development of both titles, sent code of bully to toronto to help speed up game development, and somehow the lawn mower theme was also brought over.
This is seen alot. GTA SA data is found in bully and vice versa. Some people keep saying the skateboard found in GTA SA was meant to be used in game, but i disagree. I highly think the skateboard found in the game code was actually from Bully. All 3 companies shared data code during the development of those 3 titles.
What i can say off the top of my head:
Rockstar North shared GTA SA driving and other game mechanics with Rockstar Vancouver, and Vancouver shared some unknown data with Toronto for The Warriors and all back around.
All 3 titles were in development at the same time and SA was released first, warriors second and bully third.