||LTSA & Police Myth|
|Trucks travelling at the new open road speed limit of 90 kph will be almost impossible to pass legally.
||A 90 kph speed limit for heavy trucks and 100 kph for other traffic is practicable.|
A car passing a truck travelling at 90 kph will require almost 2 kilometres of clear road ahead if the car observes the rigidly enforced 100 kph limit and the 2 second separation rule before overtaking.
The impact will be that drivers either break the law or effectively the open road speed limit will be reduced to 90 kph.
Based on Truck and 90 kph, 2 sec separation beforehand, 10 metre clearance at end, allowing for oncoming traffic at 100kph + 10% clearance of that:
90 kph = 90,000/3600 = 25 metres/sec.
100 kph = 100,000/3600 = 27.8 metres/sec
Distance to pass = (2 x 25) + (Length of Truck) + 10 + (Length of Car)
Say car is 5 metres, truck is 20 metres, then
Relative distance to pass = 50 + 20 + 10 + 5 = 85 metres
Time to pass = 85 metres / 2.8 metres/sec = 30.4 seconds
Distance travelled to pass = 27.8 * 30.4 = 844 metres
Clear distance required to oncoming traffic = (844 x 2) (+ 10%) = 1688 x 1.1
= 1,856 metres = 1.9 kilometres.
Compare this with the similar situation but overtaking at 120 kph and allowing for oncoming traffic at up to 120 kph:
120 kph = 120,000/3600 = 33.3 metres/sec
Time to pass = 85 metres / ( (33.3 - 25) = 8.3) metres/sec = 10.2 secs
Distance travelled to pass = 33.3 x 10.2 = 341 metres
Clear distance required to oncoming traffic = (341 x 2) (+10%) = 682 x 1.1
= 750 metres (allowing for oncoming traffic at 120 kph!)
The overtaking car has also to cope with the possibility that the truck will accelerate during the overtaking process.
The consequence of this law change is that few cars will be prepared and able to overtake large trucks, average speeds will fall resulting in economic loss through time wasted as well as through more accidents caused by driver frustration.