Highway police spokesman Ma Ling-wen said the speed limit on the ramp was 40 kph (25 mph) but it was not clear how fast the bus had been going.
According to the fire department, of the 44 local people on the bus, 32 were pronounced dead at the scene, including two who had been hospitalized, and the other 13 were still being treated in nearby hospitals around Taipei for serious injuries.
It is believed most of the poeple on board were local elderly tourists returning from a cherry blossom tour in Taichung, and police confirmed no foreign nationals were involved. Authorities deployed 20 ambulances to the scene to help with the rescue. Investigators said driver fatigue could not be ruled out, the Apple Daily reported.More news: Google uses Valentine's Day doodle to bring awareness to endangered animal
Tsai said the authorities must "make all efforts to save the injured and properly handle the aftermath of the crash".
Tragedy struck in Taiwan on Monday night when a tour bus crash killed at least 32 people.
The crash is the latest accident involving tour buses in Taiwan.More news: 'Mass Effect: Andromeda' to have meaningful sidequests, akin to The Witcher
Another coach carrying mainland Chinese tourists crashed into a bridge in southern Taiwan earlier this month, injuring passengers.
It comes after a bus inferno in July a year ago killed 25 Chinese holidaymakers on board, and is the latest in a series of deadly incidents that have tarnished Taiwan's safety record.More news: Mitsubishi names new SUV 'Eclipse Cross' ahead of Geneva debut