He told the inquest jury at Sunderland Magistrates' Court that he was responding to a 999 call and had his blue lights flashing and was sounding the siren.
Mr Reed said he was convinced Gareth, who had celebrated his 18th birthday two days earlier, had seen him and stepped back from the road.
He said was travelling at 40 to 45mph across the bridge to a fire in Southwick, at about 9.15pm on March 13, last year, when the accident happened.
Mr Reed said: "The blue lights were flashing and my headlights were on, so it was clear in my mind that he had seen me.
"He stepped back and was in the middle of the pavement.
"Then he decided, without any warning, to cross the road by running from a standstill."
Unemployed Mr Tuckwell, of Westheath Avenue, Grangetown, Sunderland, had been arguing with his 16-yearold girlfriend and another woman before the accident.
It was the day before he was due to move to the Midlands to start a building job with his cousins.
A post-mortem examination confirmed he died as a result of an injury to the back of his head when he hit the road.
Toxicology reports revealed he had been drinking and would have been about twice the legal drink-drive limit.
Accident investigator PC Colin Newman said: "He has stepped out into the road and left the driver no time to react.
"His judgement may have been impaired by the amount of alcohol he had drunk.
"He was either unaware of the engine or has failed to judge its speed, despite the fact that lights were flashing and the siren was being sounded."
The jury returned a unanimous verdict of accidental death. After the hearing, Gareth's father, Michael Tuckwell, 43, said: "He was a typical teenager and was very happy as he had just had his birthday and was looking forward to starting a job.
"He was happy-go-lucky, and anyone who met him would never have forgotten him."
Written by Northern Echo
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