Matthew Daley, 34, of Worthing, is being held on suspicion of murdering Don Lock, 79,  and police have until midnight to charge or release him.
Mr Lock died from stab wounds after a suspected road-rage confrontation near Findon, which Sussex Police described as an "unprovoked attack".
On Saturday a knife was recovered and sent for tests.
His family gave an emotional news conference on the same day to pay tribute to the man they described as "gentle, kind and loving".
Mr Lock's son, Andrew, said the family had been "incredibly overwhelmed" by messages of support and they were "deeply grateful" to the people who had tried to treat Mr Lock at the side of the road.
The great-grandfather had recently celebrated his 55th wedding anniversary with his wife  Maureen and had also been given the all-clear from cancer.
He was attacked after getting out of his car following a crash with another vehicle on the A24 on Thursday evening.
Paramedics were called but he died at the scene.
On Saturday Det Ch Insp Paul Rymarz said Mr Lock was the victim of a "violent, unprovoked attack that he had no chance to defend himself against".
He described the investigation as a "fast moving operation" and renewed his appeal for anyone with further information to contact the police.
Detectives are continuing to question a man about a fatal stabbing after a car crash in West Sussex.