In June 2010, Lillian Groves was playing outside her home when her ball rolled out into the street.
The 14-year-old, from New Addington, Croydon, looked for traffic before she went to collect it but was hit by a car and suffered catastrophic injuries.
She died in hospital later that night at roughly the same time that John Page, the speeding driver who hit her, admitted smoking cannabis before getting behind the wheel.
Yet the schoolgirl's family only learned a cannabis joint had been in his car on the day he was sentenced.
Police in the UK currently lack an equivalent drug-testing device to the breathalyser, meaning Page did not undergo a blood test until nine hours after the crash.
That was far too late for officers to prove – as required by law – the drug had caused impairment.
I first met Lillian's family on the morning of her death – handing her grandmother a note at their doorstep, before abiding by their request to be allowed time.
A few days later I was invited back to their home where her parents, Gary and Natasha, described the agony they felt at the loss of their "little princess".
We remained in regular contact. When I interviewed them after Page received an eight-month sentence (he was released within weeks), their despair had turned to anger and frustration.
That week's article ran with their concerns and the following Monday I suggested to my editor the paper should launch a campaign calling for the Government to change the law.
It was an ambitious goal, but he required no convincing. Our news editor suggested a name – Lillian's Law.
When I called the family, they were sold straight away. The launch article ran with the headline: 'Sign our petition and help catch drivers who take drugs".
And people did, in their thousands.
Croydon Central MP Gavin Barwell was the first politician to back the campaign, followed by Mayor of London Boris Johnson.
I contacted road safety charities, got the backing of international drugs-testing companies, and wrote about the out-of-date balance-and coordination tests currently available to police.
In October 2011, Barwell raised the campaign at Prime Minister's Questions, prompting our first big breakthrough – an invitation to meet David Cameron at Number 10. It led to an exclusive story for us when he promised to revise the law.
By then, the campaign had become bigger than us – the family were interviewed by every newspaper and TV channel.
Following our meeting with Cameron, the Department for Transport announced the creation of a panel of experts to explore a new law.
Then in January, beyond all our expectations, Cameron hinted the legislation could be included in the next Queen's Speech.
And last month it was. The Crime and Courts Bill will make it an automatic offence to drive under the influence of drugs if it becomes an Act.
The Advertiser gave Lillian's family a platform, but it was their determination which did it. It feels brilliant to be part of that.
This article first appeared in Press Gazette – Journalism Weekly. Click here to register for your free copy