Two motorcyclists died in separate crashes during a night of carnage on the roads.
A 54-year-old Brighton man was killed after his Honda Deuville and a Honda Civic smashed head-on in Coldean Lane, Brighton, around 5.20pm on Monday.
Rescuers battled to save his life at the scene but he was later declared dead at the Royal Sussex County Hospital.
The 20-year-old male driver of the Honda Civic was not thought to have been injured in the crash.
Three people from Brighton have been arrested following the crash and were still in police custody yesterday.
Sergeant Mark Greer, of Brighton and Hove road policing department, said: "It was a tragic accident.
"We are doing everything we can to investigate this.
"We would like to hear from any witnesses who have not yet contacted the police."
Five-and-a-half hours later a 16-year-old riding a Peugeot Ludix moped was involved in a collision with a VW Golf on the Shoreham flyover section of the A27.
The teenager, thought to have been local, was unconscious when paramedics arrived and was pronounced dead at the scene.
The driver of the Golf, a 51-year-old woman, also from the area, was taken by ambulance to Worthing Hospital where she was treated for shock.
Janine Bell, the spokeswoman for Sussex Ambulance, said: "Unfortunately, road traffic accidents involving motorcyclists are not that unusual.
"But to have two fatal incidents involving motorcyclists in a day in the space of five hours is unusual, particularly as the weather conditions were not poor. The roads were not icy. It is very sad."
None of the casualties has been identified by police.
Anyone with information about the Coldean Lane incident should call Inspector Jim Bartlett at Brighton and Hove road policing department on 0845 6070999.
Anyone with information about the Shoreham flyover incident should call PC Russell Weekes at Chichester road policing department on 0845 6070999.
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