A driver is 100% responsible for deciding what is a safe speed and braking distance for his vehicle.
Just one example: This evening I collected several packages of drinking water from Makro (around 500 litres, 500kg) in my truck.
This load makes a considerable difference to my braking distance, and I have to drive accordingly - leave a bigger gap and start to decelerate earlier.
The driver may not have been "speeding" (i.e. - Exceeding the signed speed limit) but, very clearly (as he hit a stationary vehicle at a red traffic light), he was driving faster than it was safe to do so.