Mercer explained that if he slowed down he could have been stuck between the two cars which could also have caused an accident.
"If I were to slam the brakes my car could've spun around into the traffic from the other direction, because of that I had to accelerate to make sure I got through uninjured
or, there's no accident.
"I had to make a snap decision so I'd rather take the safe option which results in no one dying than, yeah, having an accident."
The constable who issued the ticket was reluctant to speculate on Mr Mercer's suggestions as to the dangers of him braking hard instead of speeding up, but otherwise did not refute Mercer's case, the court heard.
Justice Brewer said Mercer's evidence was that he exceeded the speed limit so as to avoid death or injury.
"There is no evidence to the contrary. It is not necessary for Mr Mercer to prove his act was objectively necessary to avoid death or injury, just that his act was taken [in that he took it for the purpose] to avoid death or injury."
In reaching his conclusion, Justice Brewer said driving was a "dynamic activity".
He pointed to Land Transport (Road User) 2004 Rule 1.8 which provides a general exception that a person was not in breach if they prove that the act or omission complained of took place in response to a situation on a road and the act or omission was taken to avoid the death or injury of a person.
"Conditions can change suddenly, and drivers have to react to them suddenly. Rule 1.8 recognises this and excuses [in this case] exceeding the speed limit so long as the person charged proves the criteria in the sub-clauses."
He also said District Court Judge Blackie "erred in not giving reasons as to why Mercer's explanation did not amount to a defence to the infringement notice".
While Judge Blackie acknowledged Mercer's argument, he did not address it.
"By his decision the Judge rejected the argument, but he gave no reasons for doing so," Justice Brewer said.
"In light of my analysis of the evidence I have concluded that led to a miscarriage of justice. The appeal is allowed. The infringement notice is dismissed."
Mercer represented himself in court.