The female driver, from Horsham, sadly died at the scene when her Ford Fiesta car was in collision with a stolen black Mercedes AMG estate as it joined the northbound carriageway on the A24 on the Farthings Hill roundabout near Broadbridge Heath shortly after 2.30pm.
A passenger in the Fiesta, a female friend of the driver, also 70 and from Horsham, suffered minor injuries.
According to Sussex Police, officers were quickly on the scene and were joined in the search for the driver who fled from the scene by dog units and the National Air Support Service.
A forensic examination has been made of the scene between the Broadbridge Heath and Robin Hood roundabouts.
Earlier, at 2.19pm, officers had traced the black Mercedes, which was believed to have been stolen in a burglary at a house in Goring, at the Buck Barn crossroads. It was seen to leave the services in convoy with a Mercedes A Class.
Both vehicles failed to stop for officers and at 2.23pm the Mercedes A Class, which had also been stolen, crashed into a hedge near the services. The driver ran off, police said, but was located and arrested nearby.
The second car, the stolen Mercedes AMG, continued to drive away, but was not being pursued by police when it was involved in the fatal crash 11 minutes later.
Detective Inspector Will Rolls, said: “This is a tragic incident involving two innocent members of the public. We offer our heartfelt sympathies to their families and specially trained officers are providing support.
“Enquiries are ongoing to identify the driver who ran from the scene after crashing in to the Ford Fiesta. We believe he tried to flag down a lift from near the fatal crash scene minutes later.
“I would appeal to anyone who has any information to contact us online or call 101 quoting Op Mantra, serial 684 of 10/11. If you have urgent information about the other driver, dial 999 immediately.”
An 18-year-old man from Feltham has been arrested on suspicion of burglary.
The Independent Police Complaints Commission has been notified because of earlier contact with police.
The A24 has now reopened.