Wednesday, September 23, 2015

Tackling congestion as an economic, not engineering, problem

In my previous post, I mentioned that congestion should be tackled as an economic problem rather than an engineering problem. I think this deserved to have its own post to explain what I meant by that, because it's a very important issue to my mind.

The engineering approach

The typical approach of seeing congestion or vehicle traffic as an engineer problem is supposing that vehicle flows are essentially static, a known input into the model. XXXX vehicles per hours come at the intersection by the south and then go through the intersection towards the north. This value is taken as a constant: no matter how congested the intersection, the same number of people will want to make this movement, at the same vehicle flow. So in that context, all that matters is making sure that the road can take that traffic flow in an acceptable manner.
The engineering approach to intersection design: traffic flows are an input that once determined is never modified
So that's an engineering approach, simply assume certain traffic flows and then design the road network to accommodate that "load".

The economic/behavioral approach

The engineering approach is very simplistic because it essentially eliminates humans from the problem, to the point that human beings could simply be absent, the model cares only about vehicle trips. It doesn't ask if having so many trips is good or bad or if these could be reduced, it considers these to be natural constants that must simply be dealt with rather than a sum of individual decisions, decisions that can change if the context changes. The economic approach expands upon the engineering approach by looking at decisions that people make, trying to see why they make it and what alternatives they have.

In the end, a decision to go somewhere can be modelized as an economic decision which has costs and benefits (or "utility" as microeconomists put it if I remember correctly). 

Benefits or utility of a trip is generally the desired objective of the trip. The distance traveled is NOT a benefit, there is usually no direct benefit from traveling a certain distance, except on joy rides or pleasure strolls. If you have a convenience store 2 km away and another 10 km away with the same goods at the same price, you will go to the nearest one all the time. Benefits can be of many kinds, for example, the benefit of commuting to work is the income one gets from doing the work. If someone earns 100$ per working day, this can be assumed to be the benefit of the trip to and from the workplace. If he goes to a store or a restaurant, it is the utility of the good or meal he expects to purchase. Etc...

Costs include, of course, the actual monetary cost of the trip, especially marginal costs (costs that are directly incurred by the trip), like gas, parking fees, tolls, transit fare, etc... Fixed costs may or may not be considered, depending on the analysis made, for example, someone who has a car may consider only the marginal cost (gas, maintenance) for individual trips, but in deciding whether to buy a car or not for commuting to work, fixed costs may be considered fully. Costs also include the time that the trip takes, as the time is "spent". That is where speed becomes relevant, by potentially reducing travel time, and thus, the cost of the trip (reducing distance can achieve the same thing). On the other hand, congestion, by lowering speed, increases travel costs in terms of time.

Also, the cost of a minute may vary depending on comfort or perceived safety. For example, biking 10 minutes on a cycle track may be less "costly" in people's minds than 10 minutes on a road with a lot of fast-moving traffic, or 10 minutes on a free-flowing highway may be less "expensive" than 10 minutes sitting in stop-and-go traffic.

So the decision process of an individual measures in some way the expected benefits from a trip, versus the cost of the trip. If the trip's expected costs are higher than the expected utility, then people will then generally not make it. But they also have different alternatives: cars, transit, biking, walking, etc... So people examine these alternatives and select the one they think is best.

Example of mode comparison for one trip, all modes except walking makes the trip worth it, though using a car is most expensive, as it saves time, it has the highest benefit/cost ratio

Now, of course people don't bother making these calculations to the cent, but they do some estimates in their minds on some level. Everyone is different, and if you summed up everyone's thresholds together, you would obtain something like a demand curve:
Hypothetical example of traffic flow "demand curve" showing the traffic flow depending on the cost of making vehicle trips, as congestion increases, or as the cost of travel increases, some people may find other paths, delay trips or shift to other modes like transit, reducing vehicle trips.

An economic approach to solving traffic/congestion issues would recognize and consider this reasoning. Even the delays provided by the simulation would have a feedback effect on traffic flows and require iterative simulations to achieve a plausible result. As the level of service gets worse, the cost of travel at that point in time increases, encouraging people making elective trips (recreative trips, non-urgent trips, etc...) to temporally displace their trips, or encouraging drivers to find new paths or choosing different modes of transport.

As a traffic engineer who has seen many construction projects come and go, I have seen this process in real life. When a project gets under way, the first few days are the worst, but then people adapt to the new situation by trying new paths or changing their habits in a "trial and error" manner and things slowly get better until a balance is reached.

Considering an economic approach also opens the door to a lot more avenues to solve traffic issues than just increasing road capacity (increasing road supply), you can also implement policies to manage demand. Essentially, the economic approach sees congestion as a matter of demand (number of vehicle trips) exceeding supply (road capacity), which can be compensated either by increasing supply (increasing road capacity) or increasing the price of using roads (through higher travel times and/or direct monetary costs) to decrease demand, rather than merely a matter of a static load applied to a fixed system.


Visually, the process becomes much more complicated with more feedback loops:
Engineering process revised to include an economic approach to congestion
One of the reasons why engineers don't like this vision, apart from the added complexity, is the "CHOOSE" part, and the availability of non-engineering policy solutions. Engineers are professionals taught to avoid partiality and who prefer to be neutral experts rather than militants for implementing given social policies, so considering non-engineering solutions makes them feel very ill at ease as they feel it exceeds their job mandate. At the same time, politicians who don't have much vision may simply ask engineers to solve congestion issues, entrusting experts with solving their city's problems. This may create a situation where economic solutions are not considered as the experts asked to study the situation do not think they have been given the mandate to evaluate these policy solutions.

Some ideas of an economic approach to congestion

OK, so there are alternatives to engineering solutions to deal with traffic demand, but what are they exactly? On an economic basis, there are three kinds of reductions of vehicle trips that can be achieved.

  1. Spatial and temporal redistribution of trips: in that case, certain trips are delayed or hastened, or go through alternative roads that are less congested. When the costs of congested paths during peak hours become relatively higher than alternatives, then people are less likely to opt for them. This is most effective for elective trips without a fixed schedule or when alternative paths exist, which is not always the case (ex: bridges).
  2. Elimination of trips: when costs of trips increase, the cost may become higher even than the expected individual benefit of the trip, in which case, the trip can be canceled. In this case, what brings about the change is not relative cost, but the absolute cost of the trip that gets too high for it. This may be a problem for the local economy, but it may also favor the replacement of regional trips by local trips, which may be good for local communities. Commutes are least sensitive to that possibility, because benefits tend to be very high, and during congested times, commutes tend to be the majority of trips, so the ability of this approach of reducing congestion during peak hours is small.
  3. Favoring travel modes that are more space-efficient: walking, biking and transit all use a lot less space than single-occupancy vehicle. For example, a simple bus line with 12 trips per hour (1 per 5 minutes) can increase passenger throughput by 70% (700 people per hour versus 1 000 people per hour for a regular street lane) while using only 1% of that lane's hourly vehicle capacity. In this case too, the effect comes from relative cost, not absolute cost.

I will provide a few examples of different practical ways of achieving this, ideally based on real-life examples. But first, I want to address something. The idea of regulating or influencing individual behavior through policies based on economics often leads to accusations of social engineering. I just want to respond that neutrality in this case is essentially impossible. Streets and roads, by their very design, are public goods except for a few exceptions, as such, funding for them is determined by the public authority responsible for them. Therefore, that public authority is forced to make a choice, and the choice it makes will necessarily affect users' behavior and consumption of that good.

The default in North America of having public entities take care of roads, widening them when needed and being funded through taxes is an approach that is just as much social engineering as an approach that would have drivers paying tolls and fees with no income from taxes. Allowing people to park for free on the street and mandating high numbers of off-street parking for every development is just as much social engineering as charging street parking 5$ an hour and limiting street parking.

So accusations of social engineering sound hollow. It is impossible for policy to be wholly impartial on that matter. The closest thing would be a complete market-based solution. Hint: that market-based solution would not look anything like current practices.

Letting congestion take care of it

This is the solution that is most often applied, but rarely intentionally. Increasing vehicle capacity is often very expensive and politically contentious in urban areas due to expropriations, so when government budgets are tight or governments are weak, projects to increase road capacity to deal with predicted increases in population of a metro area may be put on ice.

The reasoning of the approach is the following: higher traffic demand increases road saturation and congestion, congestion increases travel time for vehicles on the road, travel time is part of travel cost, so travel cost increases, discouraging trips and so reducing demand. This is the flip side of the "induced demand" theory.

So, is this a good solution? Not really, at least, not on its own.

The biggest problem is that roads do have a major economic role to play in the modern world, for freight and for rapid movement of public and private services. Congestion is much worse for these vehicles than for commuters, because they delay the provision of services and may slow certain economic activities. Congestion can also be disastrous for transit, the alternative mode that is best placed to replace SOV trips during congested commute periods as high demand periods result in higher frequencies (making transit best in peak hours and worse in off-peak periods, the opposite of private vehicles). The slower transit vehicles go, the more expensive they are to run, so if transit runs in mixed traffic, congestion can be disastrous for it. That is what killed the streetcars.

Finally, in terms of development incentives, congestion can lead to sprawl as industries and other developments flee congested areas. Since congestion is rare outside of peak hours, commercial developments are much less affected by it, so having congested arterials do nothing to keep commercial areas in the heart of residential developments.

So, to sum up, developing a greater tolerance for congestion is not bad, but relying exclusively on congestion to regulate traffic is a bad approach.

Congestion charge

The congestion charge is a relatively recent new development that has been implemented in many cities like Stockholm, London and Milan. The basic idea is to put a price on congestion by charging drivers of vehicles entering a congested zone a certain price. This creates a financial incentive to avoid this congested area, that is most often limited to the center of the metropolitan area.
Schematic example of a congestion charge system, every vehicle crossing the dotted red line gets charged a certain amount
This system has been shown to work very well to lower congestion in central areas. It makes commuting by car much more expensive and thus helps make transit and other modes of travel far more attractive, relatively speaking. It also keeps these areas with good enough traffic flows to allow trucks and service vehicles to move easily inside the city and it provides a new revenue source to fund transit and other infrastructure projects.

What price to charge? It can be modulated to obtain the desired congestion level. The higher the charge, the fewer vehicles there will be. If we ignore political realities, we could sum up a gradual process that could be called the EASY SOLUTION TO CONGESTION:
The Easy Solution to Congestion... I'm only partly facetious, it's politically suicidal, but it would work

However, it has its issue. For one thing, it can be very hard and expensive to do, because creating such a zone in a heavily gridded city means there is likely to be dozens of streets through which vehicles can enter the zone, and if only the major arterials are controlled, you can push vehicles to use detours on residential streets to avoid paying the charge. If the city is on an island or separated by rivers and there are a few bridges only to connect to the suburbs, it can be much more easily done. 


A theoretical congestion charge zone for Montréal, relatively easy to do thanks to the location of the city on an island...
...but in Toronto, it would be extremely complicated to apply because there is no obvious boundary and any border would cut through a lot of streets, including small side streets
If the city is at the center of the region's highway system, a congestion charge may also make it hard for people to move from one suburb to another, which would generate a lot of backlash.

Finally, the biggest problem with the congestion charge is that, just like letting congestion do the job on its own, it concentrates its effects on the city itself while leaving suburbs unaffected, or less affected. As a result, if the city is not a big enough draw on developments already, it can instead hurt it, as industries and offices flee outside of the congestion charge zone to avoid employees and trucks paying the charge. So it can be a sprawl incentive as only people who live and/or work inside the city are affected by the charge. So, in effect, it reduces car use inside the city, but it may push developments outside the city.

Tolling high-speed roadways

This is an approach I've often spoken in favor of and which can be seen as an user-payer approach, as high-speed roadways are very expensive to build, and the current way of funding them through licensing fees and gas taxes subsidizes suburban sprawl by making cities pay more than their fair share of high-speed roadways. Tolling highways have the secondary effect of making long travel trips much more expensive, which is the reverse of a gas-tax funded highway which both saves users time (being faster) and money (as they use less gas per distance traveled), a practice that enables people to live much farther than they would otherwise. With tolls, this imbalance can be somewhat corrected, and with tolls being an upfront, marginal cost, the impact is much greater than a yearly fee or tax.

For example, here is a theoretical comparison between 5 trip choices, supposing a time value of 20$ per hour:
  1. A commute from a suburb 20 km away on an highway, without toll
  2. A 10-km commute from an inner suburb on lower-speed roads
  3. The first 20 km commute but with a 10 cents/km cost added
  4. A 10-km bus commute with a monthly pass (so no marginal cost)
  5. A 20-km bus commute with a monthly pass also
Comparison of different scenarios
Without toll, the suburban highway commute is in fact the cheapest because it is the fastest, compensating its slightly higher marginal cost (gas+maintenance). So it's very tempting for someone to go live further away to benefit from cheaper housing prices, and once there, the 20-km bus commute is incredibly long and unpractical, even if its marginal cost is nil, so that the odds of people residing there using transit are very low.

Once you add a toll to the highway, the 10-km car commute becomes the cheapest option. The actual monetary cost (excluding time) is also 3 times higher than with the car commute, so that the annual cost, just in variable costs, is about 1 500$ higher for the 20-km highway commuter over the 10-km urban car commuter. That is not a negligible difference.

A great thing with this approach over the congestion charge approach is that tolling highways increases the cost of ALL long distance trips, and not just trips to the city core. So people can't simply escape the fees by moving to suburbs and finding jobs in other suburbs.
With a congested central city and/or a congestion charge, developments (homes AND jobs) tend to flee to suburbs to avoid the congestion in the center, favoring inter-suburb trips
With tolled highways, inter-suburb trips are just as expensive as working downtown, so developments tend to gather in nodes to reduce distances, thus reducing or eliminating the need to use highways for many residents

Building low-cost, high-capacity, low-speed road networks

This is totally different philosophy to building road networks. Some urbanists think that road capacity induces demand, I disagree strongly, it is road SPEED that induces demand, because it lowers travel costs and makes it more affordable to travel longer distances. By that reasoning, a strong grid of low-speed roads would yield high capacity, allowing freight and service vehicles to move reasonably fast, while not inducing too much vehicle demand.

Examples of this approach can be found in Japanese and European cities, where high-speed roadways skirt around urban areas, but with a high number of major streets in cities that have low speed limits.
Munich, Germany, the highways are in orange, see how they circle the city but do not enter it, average speed on urban arterials without congestion, according to Google Maps, is about 25-30 km/h, 15 to 20 mph
Sendai in Japan, another example of highways avoiding the urban area, but with a strong grid of arterials inside the city, according to Google Maps, without congestion, the average car speed on these arterials is 25 km/h, 15 mph
The advantage of such a system is that low speeds keep vehicle demand relatively low as alternatives like transit and bikes are more competitive with cars on city streets. It also makes far-flung suburbs disconnected from the downtown area because the highways fall well short of it, forcing suburbans to travel half the city on slow arterials to get where they want to go inside it.

The problem of this approach is a risk of the creation of a semi-independent edge city around the highways that skirt the town if no measure (toll, planning) is taken to prevent it, which will eventually congest that peripheral highway, complicating long-distance trips and road freight much.

Now, I know what some may say: this is all fine and good, but that option seems to be valid only if we start from scratch what if we already have a hierarchical road network with plenty of highways inside our metro areas?

The answer is that you can gradually transfer over to that system by converting highways to boulevards like some cities did, and applying traffic-calming measures on major arterials to slow traffic down.

Limiting parking or increasing parking costs

Parking is also a crucial method of solving congestion issues with economics. Indeed, every SOV trip begins and ends with a parking spot, so if there is none in close proximity to the destination, that is a major deterrent to vehicle trips. Any parking fee also becomes part of the cost of traveling with a personal vehicle. So if parking is expensive, then it provides a strong incentive to find another way to get to the destination. If parking is limited, then it imposes a hard limit on the number of people who can come there by car, though in that instance too parking prices should increase a lot, as a way of rationing parking spaces.

As far as examples go, Calgary is an example of a city that let downtown parking prices rise very high. The average downtown parking spot going at over 450$ per month, which is very high for a city with just over 1 million people. Ottawa is an example of maximum parking limits, especially downtown. The result of Ottawa's policy is a very high transit use by downtown commuters, that some credit to Ottawa's BRT... except that the high transit use was present long before the BRT, which opening is not associated with any increase in transit use.

The same can go for residential parking, which is even more crucial, because if people do not have a place to park their car at home, they can't even own one, and if they can't own one, they can't use a car to get around, at least not regularly. The presence of on-street parking can short-circuit this process as people may simply park on the street without having to pay for a spot, which is why residential parking permits make a lot of sense.

On a tangent, many cities currently limit population and job densities in areas to keep vehicle traffic at a manageable levels. Except that is based on the supposition that, no matter what. people are going to travel by cars, or maybe that being able to own a car and to use it is a basic right. In fact, people have different ways of getting around, and those without cars are even forced to find other ways of getting around. So if you want to limit vehicle trips to avoid congesting the road network, instead of restricting population/job density, why not restrict vehicle density by restricting the number of parking spots available? That way, you can restrict vehicle trips without actually restricting development unduly.

Anyway, limiting parking or having higher parking fees is a bit like a congestion charge as in they mainly target people who go downtown or any dense areas, because in lower density areas, parking is likely to be more abundant and affordable, if not outright free.

Invest in rapid transit

If you look at the economic comparison between modes above, you can notice that regular local buses do very poorly in the comparison, even compared to commutes twice as far on tolled highways. To make transit more attractive, you can try to reduce the cost of tickets, but that is not why transit is not used that much in general. With monthly passes, the marginal cost of transit use is even 0$, so it's hard to see how you could do better by subsidizing transit more. The issue is the cost of transit use in time, so to attract people to transit, it's best to have rapid transit.
Rapid transit makes transit competitive in terms of marginal cost, which local buses generally fail to do
Of course, for rapid transit to be really useful, you need to connect it to concentrations of jobs and homes, and gather developments around it, to optimize the investment. It's also important for rapid transit to go BEYOND the current limits of developments to really go through the entire metropolitan region. This reduces transfers (and every transfer increases travel time by 5-10 minutes) and helps shape developments in suburbs to be built around transit rather than around highways. Some people may say that building rapid transit is too expensive to do that, but remember that once in low-density areas, you can build rapid transit on the surface for little more than you'd pay to build the highways that are likely already there.

Conclusion

OK, so this was a very long post, but I think it gives a good idea of ways to deal with congestion that doesn't entail a ponzi-like ever-escalating flurry of road building and rebuilding. Of course, all of these approaches are politically hard to apply, when people are used to having something to use for free for so long, any change that has them paying more, even if fully justified, will be a hard sell. However, unless we agree to be honest about our choices, these alternatives will never be properly discussed and considered.

Friday, September 4, 2015

The idiocy of traffic studies

Traffic studies, or traffic impact assessments, are a big part of the job of a traffic engineer. They are an oft-forgotten part of how urban developments occur, but they should not be forgotten, because they are one of the worst obstacles there are against denser, less car-centric developments. I've seen first hand how they worked.

The idea is that before any development occurs, a traffic study must be made to predict the impacts of this new development on the current road network. The traffic study generally goes down this way:
Here is an area to be studied, in gray, the existing developed zones, the yellow lines are the major arterial network and the red lines are an highway
1- Describe the proposed developments: how many houses, how many stores, how big are the buildings, etc...
New residential and commercial developments proposed by developers and/or city planners
2-Describe the current "level of service" of major intersections around the development during peak hours, meaning evaluating the congestion level and the delays for vehicles at them. Levels of service are rated A to F, with A having the lowest delays and F, the highest delays.
Current levels of service of intersections during worst peak hour
3-Based on the proposed development, generate likely trips for these new developments and distribute them on the network
New trips generated or attracted by the new developments and where they go
4- Based on these new trips, estimate the new level of service and congestion level of the road system
Expected levels of service at intersections based on projected traffic levels, with unacceptably high congestion (F and E) at two intersections to the west of the highway)
5- If these levels are unsatisfactory, then propose interventions on the bottlenecks of the network to reestablish acceptable levels of service, interventions usually paid for by developers
Interventions (usually adding lanes to certain approaches) reestablish acceptable levels of service on the intersections with levels of service worse than D
So that's how traffic studies are usually done. Seems sensible, doesn't it? You catch congestion upstream and adapt the roads for upcoming demand, making new developments pay. Well, let me count down the ways in which it this approach is terrible.

Assumptions, "conservative" approach and overdesign

The first issue when designing a traffic study is determining the number of trips generated by new developments. How can one proceed in this? Well, like all predictions, this is based on certain assumptions, overt or hidden, and past experience.

The bible here is the ITE's Trip Generation handbooks, which collects vehicle trip data from various locations depending on land use, then presents the result in the form of graph, based on a rate of trip per assumed relevant characteristics, for example, per 1000 square feet of Gross Floor Area.

By using that data as is, there are a flurry of assumptions that are made:
  1. All locations of a given land use with similar floor area (or other quantitative feature) will have a similar number of trips generated, it doesn't matter if it's a restaurant in the heart of a city or in a tiny town in the boondocks.
  2. Essentially all trips generated or attracted by the location will be made in cars.
  3. Congestion level and traffic conditions will have no effect on the number of trips made to and from that location.
Now, people may point out that these assumptions aren't always, or even frequently, true, but these still form the basis of most traffic studies. Why? Well, engineers are taught during their formation that being "conservative" with their calculations is always the best option. In this case, this has nothing to do with politics, it means always opting for more precaution, supposing higher loads than is plausible, so that the resulting design is sure to be able to withstand much bigger loads than it can be expected to face. This is rooted in structural engineering: someone who designs a bridge wants to avoid a catastrophic failure of the bridge, and so will overdesign the bridge for greater loads than necessary to make sure it doesn't happen, so loads will be high-balled and material resistance will be low-balled.

That makes plenty of sense for structural engineering, where failure destroys the structure and may even put human lives at risk, so that outcome is downright unacceptable. In that case, the cost of being conservative in one's calculations is merely a higher construction cost, there is no externality in most cases. So overdesigning a bridge has no drawback, except for the higher cost.

However, the problem comes when you apply that "conservative" mentality to traffic studies. In this case, it means assuming the worst case scenario of traffic, for instance, all different buildings have their PM peak flow at the same time, all trips are made by car, and using the Peak Hour Factor to suppose that all movements achieve their maximum 15-minute vehicle flows at the same time. It also means low-balling somewhat road capacity. So you're designing roads to avoid congestion at higher traffic levels than the roads will usually see.

This approach basically views congestion, even during the peak hour of the day, as a catastrophic failure in a way similar to a bridge falling down. It also assumes that there is no externality to overdesigning an intersection, of using longer traffic signals, of having a higher number of wider lanes, wider medians, the only drawback is the cost...
Overdesigned intersection, made to absorb the worst possible traffic flows that it can see
...the reality is quite different. Huge intersections like these tend to result in high speed travel during most periods of the day when it is not congested, creating noise pollution and having the potential for very dangerous crashes. They also occupy a lot of land that could otherwise have been used for productive land uses to create value, weakening the tax base of the city. Their surplus capacity may also induce more vehicle traffic than would have happened otherwise. Finally, their huge size makes them a barrier to non-motorized travel. 

Look at the earlier image, the pedestrian crosswalks are nearly 40-meter long (130-foot). It takes the better part of a minute for a pedestrian to cross the intersection, with vehicles turning left and right into his crosswalk while doing so, because with the time needed to cross the intersection, you can bet there is no exclusive, protected pedestrian phase. As a result of this phase, the cycle of the traffic signal probably approaches 3 minutes, and so the pedestrian delay to cross the intersection will probably be around 1 and a half minute, followed by nearly a minute of crossing an hostile intersection filled with fast-moving vehicles.

The result is that such an intersection forms a barrier to pedestrian and bicycle travel. The resulting roads that accompany these intersections are often large enough to make mid-block crossing a dangerous proposition to say the least, confining pedestrians to crossing only at rare intersections, often necessitating detours taking 5-10 minutes on foot... just to get on the other side of the road.
Another example of an overdesigned intersection, with the "Michigan left" on the East-West approaches, left-turning vehicles cannot turn at the intersection, must continue through and do U-turns in the bulbs you can see to the right and left, so they can just turn right at the intersection

The overdesigned road forces residents of the mid-density housing (32 units per hectare, 12 per acre) on the bottom of the image to do a 650-meter detour to get to the store at the top, in order to gain access to a crosswalk where they can cross the road
These externalities are ignored, it is considered to be "conservative" to spend more on a road with better vehicle capacity no matter the impact on quality of life of residents, on alternative modes of transport and on the financial sustainability of developments. This approach and the assumptions that support it make traffic studies a self-fulfilling prophecy: over-designing roads while neglecting non-car travel ensures that almost all trips in the area will be made by car, confirming the initial assumptions of quasi-universal car use, because cars are the only mode of travel the road design caters to.

To be fair, the most recent Trip Generation Handbook claims to address many of these failings by establishing some guidelines to differentiate mixed use developments from traditional sprawl, but I haven't had access to it.

Pushing developments to the fringe

Since either the developer or the city has to pay for any improvements to road capacity to deal with expected new trips, and the standard approach, up to now, didn't bother to account for a reduction in vehicle trips brought about by alternative modes of transport, this creates a perverse economic incentive. Essentially, developers are going to want to target areas of the road network that are not already at capacity and develop these rather than developing in already dense areas. That way, they can minimize the likelihood of paying for improvements, and even if they can't avoid it, they can minimize these costs.

What are the locations that are most likely to be congested? Central areas of course. In most metropolitan regions, the central city is still a strong draw for jobs if nothing else, drawing people from suburbs. So the traffic on a road tends to increase as one gets closer to the center of an urban area. Even locally, developed areas in suburbs will often have more traffic as long-distance trips and local trips overlap. Older areas also tend to have narrower roads with buildings built much closer to the sidewalks, making widening the road much more difficult and expensive.

A metropolitan area where a central city is connected with smaller urban areas through highways

A likely congestion level for the major roads, red=congested, yellow=slightly congested, as a result, developments inside existing urban areas are more complicated and expensive as developers are asked to improve roads to obtain "acceptable" levels of service
So traffic impact studies provide economic incentives for developers to spread developments all across the metropolitan area, to use every bit of unused road capacity before even considering densifying existing areas, and to avoid concentrations of buildings rather than favor areas that are walking and transit friendly. That is the very definition of sprawl: low-density developments spread uniformly on a large and growing area, necessitating cars to connect different land uses.

Short-sightedness of the process ironically leads to congestion

Even in the optic of preventing congestion, this process is a failure. The reason is that traffic studies only address the local impacts of traffic in most cases. However, due to the sprawling nature of developments and the presence of highways, it is quite likely that the majority of trips generated or attracted by a development will not come from the area considered by the traffic study.
In case there is a new development in the yellow area, the traffic study may be limited in scope to the area in the red box
The area in a larger context, the red box is in the top left, with many trips coming from or going to highways to connect to other neighborhoods
So though the traffic study may help avoid local congestion around the development, it will only channel more and more cars onto the major regional roads, which are not considered during that process. These major roads, usually administrated by DOTs or Transport Ministries, are often the only rapid links available to connect people with the rest of the region and will in essence be treated as car sewers by developers and planners, more and more cars will be fed to them, which eventually results in congestion. Of course, congestion of major regional roadways that are vital for freight transport inside a region is a major issue. As a consequence, these State/provincial entities will have to plan for more and more interventions on their roads to keep increasing capacity and deal with all the traffic cities bring into them. The result is, well...
...this. This particular right-of-way is nearly 500 feet wide.
If developments were instead allowed in already dense areas, since distances to stores and jobs are less and population density is higher, people who went to live in these places would be much less likely to use cars to get around. Since population growth in a metropolitan region is independent of developments, this means that if you force new residents to settle in car-dependent areas, to avoid local congestion in existing built areas, then you generate more and more congestion on the regional roadways which are vital to keep the region working because everything is so spread out. These roadways also happen to be at the charge of the State or province, so the cost of any capacity increase falls on all taxpayers, subsidizing far-flung suburbs who end up paying neither for local road improvements (largely paid by developers) nor for regional roads' improvement and upkeep (paid for by higher levels of governments.

So this is the lunacy of neighborhoods pushing out new developments because of fear of "congestion"... while the developments have to happen to house a growing population, so these developments are just going to happen further down the road, and the development's trips are still going to have to go through the area that refused them.

Incumbents gaming the system: impact on housing affordability

One final issue is about development economics. These studies often result in developers having to pay for interventions to prevent congestion levels from degrading, which means that the new tenants will have to pay for them through higher prices and rents. This is another way that incumbents have of using local municipal powers to give themselves certain guarantees while burdening newcomers with the price tag. The assumption is that incumbents are entitled to the same level of service they currently have, so newcomers have to pay 100% of the cost of any improvement meant to safeguard this level of service.

One car generates the same level of congestion whether it's driven by a new resident or an old-timer, so why should only newcomers have to pay to avoid the congestion generated by everyone? It is quite unfair, but that is the way that neighborhood opposition may be assuaged, at least somewhat.

From what I understand, the construction cost of new housing is vital to keeping housing costs down, even on the existing house stock. The cost of building new housing is what puts a ceiling on the price of housing, no one would pay 300 000$ for a house if they could have a similar house built for 250 000$ in the same area. The cheaper it is to build new housing, the more housing will be built, the more competition there will be and the slower prices will rise. It may even lead to older housing becoming less desirable and thus less valuable.

So when you heap on new developments the total infrastructure cost, you hike up construction costs, and as such set the stage for less affordable housing in the long run.

Conclusion

So these are my insights on traffic studies, based on my own personal involvement in them. These traffic studies only make sense in the context of sprawl, i.e. car-dependent development. In that respect, they achieve their goals in building relatively livable car-oriented areas, but they create financially unsustainable systems, increase transport needs and costs needlessly. The goal should not be to make sure that road networks are designed to allow everyone to use cars all the time to get anywhere, but to provide people with cities where they do not need to use cars and so there will be less cars on the road.

Traffic engineers need to stop considering congestion during peak hours to be "catastrophic failures". Congestion is not necessarily an engineering problem, in fact it is mainly an economic one, it is a mere shortage situation. It can be dealt with by offering alternatives to single-occupancy vehicles and by creating economic incentives to take these, either by giving preferential treatment to these modes to protect them from the effects of congestion or by directly targeting drivers through congestion fees and tolls