Norma Diffley, who ran a boutique in Donaghadee, died in the accident on the A2 dual carriageway between Bangor and Holywood close to the Devil's Elbow at Carney Hill on Monday afternoon.
The 33-year-old man is being questioned on suspicion of dangerous driving causing death.
He was arrested on Tuesday.
A 33-year-old man has been arrested in connection with a two-vehicle crash which claimed the life of a 75-year-old woman in County Down.