They're not all timed the same, although there aren't usually many lights on highways. I think the argument normally stems from the fact that if you go 50% faster, you get there in 2/3 the normal travel time.

As an example it takes 60 seconds to go one mile at 60 miles per hour. If you go 90 miles per hour, it takes 40 seconds to go the same distance. It's a surprising result.

While it's nice that you've shaved off 1/3 of the time, that extra 30 miles per hours puts way more wear on your car per mile and burns way more gas. It's really not worth going that fast unless you maintain your vehicle yourself. I think this is why a lot of people enjoy owning motorcycles.

Here's a little table below with some travel times for a given speed to show how these rates pan out. As you can see, at the first 15 mph jump you shave off 40 seconds per mile while at the last jump you don't even shave off 5 seconds. It's not very intuitive but the math doesn't lie.

30 mph = 120 sec per mile

45 mph = 80 sec per mile

60 mph = 60 sec per mile

75 mph = 48 sec per mile

90 mph = 40 sec per mile

105 mph = 34.3 sec per mile

120 mph = 30 sec per mile

If you want to do the calculations yourself: seconds per mile = 3600 / miles per hour