Space is a precious resource in a city. An underground system uses very little of it. A person driving a car at 40 km/h, however, needs an area of 60 m2.
Speed plays a role given that every vehicle needs space ahead to brake.
Moreover, private cars in the city are stationary for over 23 hours a day, while underground trains operate for around 20 hours. A single journey by an underground train replaces an average of 300 car trips. The underground system also offers urban planners the opportunity to redesign public spaces.