John Page, 37, shouted as he rode past the teenager's house in New Addington before running his finger across his throat as she looked out of the window, a court heard.
Page, who killed 14-year-old Lillian in 2010 while driving after smoking cannabis, appeared at Croydon Magistrates' Court today (Monday) charged with a public order offence.
He is alleged to have caused the 13-year-old, who can't be identified for legal reasons, 'harassment, alarm and distress'.
Joseph McKenna, prosecuting, told the court how Page cycled past the girl's house and shouted 'Oi' at 8.45pm on August 3.
The girl, who gave evidence from behind a screen, said: "When I stood by the window he was looking up at me. He pointed his right arm at me then he did a cutting motion across his throat. It made me feel scared.
Mr McKenna asked: "Were you able to make out who it was? Was it John Page?"
"Yes it was," she replied.
After Page, of Underwood, Fieldway, cycled away, the girl ran into her bathroom where her mother was taking a shower.
"All of sudden the shower curtain just flew back and my daughter was standing there screaming," she told the court.
"Her face was bright red and she was shaking from head to foot. All she kept saying was 'John, John, John'. It took me ten minutes to calm her down enough to tell me what had happened. I've never seen her like that before.
"It takes a really big man to threaten a 13-year-old girl. It's disgusting."
The court heard how the alleged incident stemmed from a "feud" between the two families.
Page, who denies the charge, says he was at home with his mother, his niece and her boyfriend at the time the alleged incident took place.
He said: "I was indoors watching television going about my usual day. I don't come out of my house very often at all."
Clearly referring to running down Lillian he added: "I was involved in something, so I stay indoors.
"I was involved in an accident a couple of years ago and since then I have suffered with going out in public and sleeping. I'm on medication and I'm undergoing further treatment."
When asked by Thalia Maragh, defending, what condition he suffers from, he replied: "Post traumatic distress. When I go outside I feel nervous and frightened a little bit. I feel people are looking at me and making comments. I very rarely go out at all."
Miss Maragh said: "What is your response to this allegation?"
Page replied: "It's total lies. I certainly wouldn't ever threaten a 13-year-old child for a start. It's a lie and that's all I can put it down to."
The trial was adjourned until Wednesday (November 21) because Page's mother, who has multiple sclerosis, was unable to attend court due to a "communications mix-up" with her son.
Page, a landscape gardener. was jailed for just eight months in July last year after pleading guilty to causing death by careless driving.
He also admitted driving without insurance when he hit Lillian Groves outside her home in Headley Drive, New Addington, in June 2010.
Traces of cannabis were found in Page's blood and a half smoked joint with his DNA on it was discovered on the dashboard of the car he had been driving.
Following his sentence Lillian's family joined forces with the Advertiser to launch Lillian's Law, a campaign that has prompted changes to drug-driving legislation.