GTA 5 DRIVING ULTIMATE TAXI CAR SIMULATOR #21

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— Warstock Cache & Carry

The Taxi is a four-door, sedan-based taxicab featured in almost every title in the Grand Theft Auto series. It is manufactured by Vapid in the HD Universe.
Distinguishable by its yellow livery and a bright "TAXI" sign mounted on the roof, it is often one of the most common vehicles on the road. At least one type or variant of taxi appears in all Grand Theft Auto series; the London Cab serves as the main taxi in both Grand Theft Auto: London games, and the Cabbie in Grand Theft Auto: Chinatown Wars.
Taxis provide a means of transport for both pedestrians and, in numerous GTA titles, the player; in all HD Universe games, the player can hail NPC-driven taxis and use them to take taxi rides to destinations, which is often a quicker alternative to driving themselves.

The player can also be used to trigger taxi driver-based missions, also appearing in almost every GTA title.

Taxi companies
The Taxi's appearance across the entire GTA series has seen various different taxi firms & companies operating them:

Borgnine Taxis (GTA III, GTA Advance, GTA Liberty City Stories; Borgnine)
Downtown Cab Co (GTA V and GTA Online)
Express Car Service (GTA IV; Roman's Taxi)
Kaufman Cabs (GTA Vice City)
L.C.C. Taxis (GTA IV and EFLC)
Mean Street Taxis (GTA III and GTA Liberty City Stories)
Sunnyside Taxis (GTA San Andreas)
Vice City Cabs (GTA Vice City and GTA Vice City Stories)