In a day when most vehicles on the road have sophisticated anti-lock brake systems with disks on each corner the amount of room you need to leave has certainly grown.
Reaction time hasn't improved but the ability to stop of many of the vehicles on the road today is pretty impressive when compared to 50 year old drum brakes.
And locking the brakes means you've lost traction on any wheel that quit turning. Braking performance is better when the tire is still adhering to the road!
