Peningkatan kendaraan bermotor yang mendadak mengakibatkan menambahnya jumlah kecelakaan.

English Translation

The sudden increase of cars is causing a large number of traffic accidents every day.

{{Menambahnya}} (increasing) is inserted in the wrong place. The Indonesian sentence means “The sudden increase in traffic is causing the increase in traffic accidents.” But it doesn’t necessarily mean the current accidents is a large number. It merely means the number of accidents increased. {{Menambahnya}} should be removed or the English sentence should be changed.
Also, “every day” is missing in the Indonesian sentence.

I think “bermotor” is also redundant. “Kendaraan” itself means “vehicles”. Another Tatoeba user is proposing a better sentence here.