It means motorists who kill while avoidably distracted could for the first time face prison.
Historically, careless driving has only been punishable with a fine, whatever the consequences.
The new offence means UK motorists, who kill after allowing themselves to lose concentration, can be imprisoned for up to five years.
So what sort of distractions might count against a driver who causes a fatal crash?
The Ministry of Justice gives these examples: reading a text message, glancing at a map, eating, drinking, tuning a radio, putting on make up.
The Transport Research Laboratory in Berkshire has a wealth of experience in analysing driver behaviour.
I was invited by government officials to visit the laboratory for a test to see how various distractions would affect my driving.
To summarise the results: my driving got worse. But was it dangerous?
Jason Ness was killed by a careless driver who was fined £575
According to Dr Nick Reed, who analysed my test, it depends when the distraction occurs:
"If it happens at the wrong time, it could cause a serious accident and the fact that you were distracted at the time could mean the difference between serious injury and a fatal accident. "
Last year the number of people killed fell below 3,000 for the first time since records began in 1926. But, still, eight people die on the roads each day.
Alongside careless drivers, the new laws will also penalise uninsured, disqualified and unlicensed motorists who kill.
According to Road Safety Minister Jim Fitzpatrick, it is hoped the number of deaths will be reduced even further.
"The government is committed to continually improving safety on Britain's roads," he said.
"Under the Road Safety Act we have already brought in tougher penalties for drivers who use hand-held mobile phones at the wheel and the introduction of these new offences today is the latest step to tackle bad driving and further improve safety on our roads."
To the distress of many families of victims, fatal crashes caused by careless driving have tended to lead to penalties no more severe than a fine.
Only offences where the higher threshold of dangerous driving can be applied have carried a potential prison sentence.
From next week careless driving in the UK could lead to prison too.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.