Analysis: Gist is that motorists complain about the traffic congestion increase from 20 years ago. The author wants to convince us that the motorists' complaints are unwarranted (not acceptable). The reasoning is that the highways have tripled capacity while automobiles registered has increased by 75% (from 20 years ago figures). Some obvious gaps are that the motorists are complaining about traffic congestion in a certain country (including all roads and streets) whereas the authors reasoning uses highways only as the support for the conclusion. Another one is that the increase in automobiles that are registered have increased by 75% however there could be other vehicles that are not registered that could be contributing to the congestion.
Also I keep in mind that to weaken the conclusion of author the correct option should give us a reason why the motorists complaining is actually warranted. Phrasing it this way helps me analyse options quicker in such type of questions:
Which of the following, if true, most seriously weakens the argument?
(A) Most automobile travel is local, and the networks of roads and streets in the country's settled areas have changed little over the last 20 years [so the travel is mostly local and the road types, different from highways as mentioned by author, have remained the same in past 20 years. This means that these roads capacity has remained virtually the same as the registered vehicles number has increased. This would be a good reason for the motorists claim if I was one of them!
(B) Gasoline prices are high, and miles traveled per car per year have not changed much over the last 20 years : Gas prices are high and miles traveled per car hasn't changed much signifies that the duration an automobile is on the road on average would also be similar to 20 years ago. This kind of supports the idea that congestion could be solved as the existing automobiles are not travelling more and thus contributing more to the congestion.
(C) The country's urban centers have well-developed public transit systems that carry most of the people who commute into those centers: This is not weakening at all, because public transit availability could mean that people shift from automobiles to public transport thus possibly reducing congestion. As the author this would be a good point to put up against the complaining motorists.
(D) The average age of automobiles registered in the country is lower now than it was 20 years ago: Average age of automobiles registered signifies the age of the automobile. How new or old it is. if the average age is lower now, it means that either the older cars have gone out of circulation (thus reducing number of cars and thus congestion) or people are just buying more new cars. However, this does not give us a clear indication of whether there are more automobiles or less.
(E) Radio stations have long been broadcasting regular traffic reports that inform motorists about traffic congestion: Informing about traffic congestion could mean either informing of presence of traffic congestion or that there is no traffic congestion. In the latter case, it would go against the motorists claims and could mean that there is less traffic congestion. As the option can give quite ambiguous outcomes, eliminated.