I have heard various assertions about just what causes traffic congestion. I have also heard various justifications of those positions. One of the most ludicrous, however, is that building highways causes traffic congestion, or its flip side that building highways eliminates traffic congestion.
Highways neither cause to solve congestion. They are a neutral factor. All they do is change the specific characteristics of the existing traffic and either reveal previously unobserved deficiencies or they provide a path around a deficiency. It may also be that they simply provide capacity through an area which was already overloaded. Other measures can have the same effects.
I should note that the “highway” I am describing here is a high speed roadway with limited accesses, usually significant fractions of a mile or more between. These accesses may be at grade or they may be grade separated to a greater or lesser extent. They key is that they are designed to move traffic at high speeds compared to surrounding roads. Simply slapping a route number or similar on an existing road does not make it a highway. Portions of a numbered route may qualify as highways under this definition while other portions of the same route do not. (If route X happens to traverse Main Street Podunkia with its traffic lights and dense cross traffic, that part of route X is not a highway even if the rural portions do qualify.)
Instead of building highways as a one size fits all solution to congestion or blaming highways for causing congestion, it is far more sensible to look at the underlying cause of congestion: traffic volume. If a road can handle X cars in the space of ten minutes, it follows that 2X cars traversing that same road in the same time will cause congestion. It further follows that removing the excess cars will remove the congestion. It also follows that upgrading the road to handle 2X cars will also remove the congestion.
The problem with congestion is that all it takes is a single choke point to increase traffic volumes leading to that choke point. For instance, a single intersecton with a stoplight can reduce the effective capacity of a road substantially if a large portion of the traffic volume must traverse that intersection. Once that intersection becomes overloaded, the traffic waiting will back up along the roadway, thus increasing congestion even though the rest of the road would ordinarily be able to handle that traffic volume easily. The obvious solution is to clear up that choke point, of course. But what happens if that choke point was serving to meter the traffic going to the subsequent potion of road which really can not handle more traffic? Fixing the first choke point simply moves the choke point further down the road. This may alleviate some problems of congestion upstream depending on specific road geometry but it does not help the congestion problem as a whole. Thus, expanding roads as a solution to congestion requires clearing up all the choke points before the real gains are seen.
There is, of course, another effect that expanding road capacity has. As the congestion on a route decreases, the opportunity cost of using that route decreases. This leads more traffic to use the newly expanded road which then drives up traffic volumes. This may bring the newly expanded choke points back to their pre-expansion congestion levels and it would then appear that the road expansion was pointless. However, that added traffic had to come from somewhere so it may be that congestion elsewhere was alleviated even though the main route that was expanded is still congested. Additionally, that main route will be exhibiting a higher throughput with potentially equivalent latency as a result of the capacity expansions.
Of course, if the added traffic on the newly expanded route is new traffic, then additional choke points can be expected and congestion becomes worse. This is a likely scenario in an area that previously had underpenetration of automobile use and the reduced opportunity cost due to added road capacity makes automobile use more attractive. In this case, it may be that the original road was many orders of magnitude undercapacity for the potential traffic volumes. In this case, it can appear that building highways makes traffic worse when, in fact, it is simply a massive infrastructure deficit that was limiting traffic artificially.
On the other side of the equation, however, we see that reducing traffic volumes also reduces congestion. This requires having viable alternatives to automobile traffic, alteratives that are good enough that a low opportunity cost due to low congestion on the roads does not persuade commuters to take to their cars. Exactly how to do this is a subject of much debate. However, there are some obvious solutions that do work.
The first is public mass transportation. In dense urban areas, this works quite well as long as the users do not have to wait too long at stations or bus stops and as long as the transit vehicles are not overloaded. The major problem mass transit faces is when density dwindles. It becomes impossible to economically provide transportation in low density suburbs as are typical in North America. This tends to lead people to drive from a pure convenience standpoint or due to a lack of other viable alternative.
Also in dense areas, pedestrian traffic is viable in most weather conditions. In fact, people will tend to walk short distances rather than drive in most cases because it is simply more efficient. In cold weather due to the fact that they do not need to wait for a car to warm up and in warm weather because it is comfortable. Bicycles (human powered!) have similar characteristics except that they need to have a safe means of sharing roads with traffic (which pedestrians already have) but they also need somewhere to be parked at destinations, unlike pedestrians. In much of North America, bicycles must share the road surfaces with automobiles which serves as a substantial disincentive.
There is, however, another way to reduce traffic in the longer term. This involves city planning. Rather than approving developments which either require or encourage automobile use (like Crossiron Mills at Balzac), require developments to be near convenient transportation links, or at least have some sort of plan to link to such transportation when it becomes available. Rather than designing massive commercial districts with absolutely no residential, mix the developments so that some fraction of workers can live near where they work.
We also need something of a cultural shift. Rather than everyone expecting to be able to buy a single family detatched house somewhere, get married, have 2.1 kids, a dog, and 3.7 cars (“the American dream”), require denser residential development. Prefer town houses, stratified condominiums, and the like instead of endless miles of single family houses. Make approvals for single family detatched houses more difficult. For larger metro areas, perhaps even a “sprawl” tax might be levied on new low density developments even though such style of taxes tends to be less effective. Tell the whiners to STFU when they complain that they cannot afford to buy a house. Eliminate the notion that eveyone is entitled to buy a house (but don’t prevent it if they can afford it). While we’re at it, plan the developments so the highest density is situated conveniently along major transportation links or is otherwise well connected.
If we can make the higher density developments pleasant and comfortable, people should have fewer objections to living in them. That includes having amenities like entertainment, health care, groceries, fitness, etc., all conveniently located. It’s not difficult to make these developments look nice. There is no need to build fascist style untilitarian architecture.
Solving the congestion problem by reducing traffic also has a significant added bonus: cleaner air. With fewer automobiles idling around, or even just running, both noxious and greenhouse emissions go down. Higher density developments are easier to heat and cool efficiently. They take up less land that can otherwise be used for agriculture, or, gasp!, nature reserves and parks. Not only would it improve air quality, but it would be beneficial from the perspective of global warming. If you don’t believe in global warming, then at least cleaner air ought to have some appeal to you.
In short, highways are not the problem. How we develop cities is.