One roadblock wouldn't work. The last thing you want is an out of control semi careening towards a wall of cars that are stopped on the road. They would need multiple roadblocks to prevent access to the Yellowhead instead, ie at every onramp. The logistics of that suddenly become a little more difficult and frankly... not doable by the EPS at the time.
I don't believe the suicide theory. If he really was aiming for vehicles it wouldn't have last as long as it did. Driving erratically from the witness accounts, does not equate to attempted suicide. For all we know, he may have been trying to dodge cars (which to some looks like aiming). I also don't believe other accounts saying it couldn't be medical because there is no way the semi would have stayed on the road that long. It is quite possible, like ken0042 mentioned, that a driver could be partially incapacitated through a medical condition. This would leave him with enough control (or the belief of control) to keep it on the road as long as he could.
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