Good analogy, if you are driving and the speed limit is 60 mph on the highway, and you drive 60 mph, but after 15 mins, your speed is required to drop to 30 mph. In this scenario (based on the first trigger),we are not even assuming/considering if there is additional traffic, only if you are going the full speed limit.
The cable company's description of their service is disingenuous.
The first trigger is only checking how much of the allotted bandwidth you are using, not that if your bandwidth is affecting other local users. This seems wrong when you are advertised to have up to speeds but don't respect the availability based on corporate self imposed limits. The only thing that should affect your speed for this trigger is network congestion, not artificial limitations.
The second trigger sounds better, except when a company like Comcast check their Cable Modem Termination System to determine throttling, they have no incentive to limit the number of users/households per system. This causes lower transfers based on the poor infrastructure implementation by Comcast.
These two triggers are rigged twice to hurt the consumer.
A better solution would have been to check the Cable Modem Termination System if it is reaching its max bandwidth or a high percent, and then have it adjust a households bandwidth for the largest users sudo dynamically. This would affect the people using the most when the resources are "truly" limited, and only limit enough to keep traffic moving. - The beauty with this plan is if they put too many people on a System at the same time, everyone's traffic will be affected. This is not good from the consumer perspective, but it would cause people to complain, hopefully leading to Comcast getting their act together and invest in better infrastructure and to not overload the connections to their system. This is standard network administration. I think you could see why the Cable companies wouldn't go with this method.
Since we are limited to the up to speed they advertise, I think that when they throttle down the bandwidth, they should throttle down the cost we pay for service until they get their act together.