Thursday, July 21, 2005

Am I there yet ?

I had to run some errands at the Mall and decided (in vain, I would add) that Sunday morning seems a good enough time. Popped in a couple of new CDs' and zipped out of my parking lot. Very soon, I was taking the ramp towards the interstate and lo and behold, what I would see is a parking lot in front of me. Yuckkkkkkks, the entire interstate on my side was turned in a three lane parking lot with cars at a virtual standstill. I felt like the boat trapped in a storm and having nowhereto run !! Finally, I managed to navigate slowly to mall (a distance of 8 miles consuming an hour). While coming back (thankfully, it was not bad, I recall Sharath's excellent blog piece on Urban sprawl and my subsequent discussions with him about the traffic congestion and what it means in the long run. Sometimes, I wonder if traffic congestion is the price we pay since we want to live in economically developed areas, closeto work and close to shopping centers.
The basic problem of our society is not congestion. It is that we want to move too many people at the same time, and that problem arises from the way society is organized here in the United States and to a large extent everywhere else also.
It is seldom possible that an area is able to create sufficient roads or other means of transportation so that everybody who wants to move at the same time can move at their desired pace. In United States, the ratio of people to cars is 1:1.2, i.e. for every person, there are 1.2 automobiles. This means that every year, we keep adding between 50 million and 60 million automobiles to the roads, but road construction increases to the tune of only 5-6% every year. The next transportation and urban planning engineers are often asked, is What can be done to tackle congestion?
Several interstate exits in and around the Washington DC metro employ HOV-2 (two or more occupants in the car at designated times) and ramp metering (a stop light allows only one or two cars to enter an expressway at a time). This helps the flow of traffic, but it often leads to massive choke-ups on the side roads because all single riders converge there.
Usage on expressway to be regulated through usage of tolls. However, this would lead to another class of haves (people who can pay the tolls) and have-nots (people who cannot). Additionally, with people who are used to using expressways for free would immediately switch to side roads leading to more traffic congestion.
Use of HOT lanes (High occupancy toll lanes). This would necessitate the construction of extra lanes next to the existing lanes on congested roads and a toll is charged for traveling at high speed on these new lanes. The DC metro is going to experiment with this approach by constructing HOT lanes on the Capital Beltway.
Building extra capacity to public transportation systems like metro, subway or light rail. DC Metro, The Washington Metropolitan Transit Authority has started preliminary work on expanding the high density Orange line to the Dulles Technology corridor. The problem with construction of public transportation systems is the high gestation period between project inception till final completion. The DC metro for example, estimates the project to end in 2012. By that time, millions of new residents would call DC their home, leading to massive traffic congestion.

Some interesting analysis is provided by System Dynamics, the study of causal inter-relationships in a system. System Dynamics provides some interesting insights into traffic congestion and the death of the mass transit systems as we know it. I recall having an excellent course in Systems Dynamics where we used a book by a MIT professor, Dr.Sterman, "Systems Thinking and Modeling for a Complex World". The Systems Dynamics analysis is adapted from this book.

Systems Dynamics : A Primer
Positive Feedback: Positive looks, denoted by R are self-reinforcing. In this case, more chickens lay more eggs, which hatch and add to the chicken population, leading to still more eggs. The + sign indicates that the effect is positively related to the cause; an increase in the chicken population causes the number of eggslai d each day to rise above what it would have been (and vice versa, a decrease in the chicken population causes egg laying to fall below what it would have been). The loop is self-reinforcing, hence the loop polarity identifier R.

Negative Feedback: Negative loops are self correcting. As the chicken population grows, various negative loops will act to the balance the chicken population. The more chickens, the more road crossings they would attempt. An increase in the chicken population causes more risky road crossings, when then brings the chicken population under control.

systems, no matter how complex, consist of networks of positive and negative feedbacks, and all dynamics arise from the interaction of these loops with one another.


The traditional solution to traffic jams and
congestion has been road building. As the number of vehicles on the roads increases, given the highway capacity, the average trip will take longer. Highway capacity can be alterd by road building, both new roads and improvements to existing roads, such as HOT lanes, toll roads, ramp metering, etc.
The model on the right can be interpreted as a series of feedback
loops, As congestion increases pressure for new roads, the Capacity expansion loop (B1) acts to reduce travel time to acceptable limits.
In the short run, the travel time falls. As people of that region notice this, they start taking more discretionary trips (loop B2). Extra miles are driven (loop B3). Over time, as driving becomes more attractive than taking public transportation, the number of cars per person rises (loop B4). Loops B2 through B4 compensate for any new road construction by increasing traffic flow. Additionally, the population of any region is not exogenous, but is affected by the accessibility of the outlying districts. As the roads expand linking the region to more areas, the "economic" size of the region grows. Because of this, people move to the suburbs (loop B5). Loop B5 is further exacberated by the fact that the local governments do not encourage high density housing, thus leading to several hundreds of single family dwellings in the suburbs. The combined effect of the four negative feedbacks B2 through B5 is to compensate strongly for any decrease in travel time caused by new roads. The model illustrates the futility of attempts to reduce traffic congestion by building new roads. Traffic always expands to fill the highways available for its travel.

Mass Transit
Options Explained Using System Dynamics
indicated in the earlier feedback loop, traffic congestion should ideally lead to more and more people using the mass transit option(s). However, there is very little Mass transit on its own can do to stem traffic congestion. As lower travel times caused by roads increases the attractiveness of driving, transit ridership drops, leading to fall in revenue. If the transit authority tries to close its deficit by Cost Cutting (loop B6), service and quality erode. Route expansion R2 operates as a vicious cycle of decreasing ridership, greater cuts, and still fewer riders. Rising fares by most transit authorities (due to federal cuts) increases the relative attractiveness of driving and more people abandon mass transit for cars. This creates loop R3.
Mass transit systems are particularly prone to self-reinforcing feedbacks because
of a high proportion of fixed costs. Attempts to build up the mass transit network to offset the positive loops that erode ridership through Mass Transit Capacity Expansion (loop B7) often fight a losing battle due to their long delays and high costs. As more and more people shift to suburbs, the locus of activity shifts outside the area served by the mass transit authorities. As population density falls, fewer and fewer people live near a bus or subway route. Public transit becomes less and less useful because You Can't Get There on the Bus, leading to still more driving and still lower mass transit ridership, in another vicious cycle, loop R4. The suburbs grow much rapidly than the addition of mass transit capacity. The spread of population into less densely populated suburbs increases the average length of trips, forming additional channels by which congestion rises to offset any gains caused by new highways. Downs, A (1992), Stuck in Traffic:Coping with peak hour traffic congestion. Washington DC, Brookings Institute advocated a seminal study which showed that the solution to reduce traffic congestion is in charging tools, reducing speed limits and installing flow restricting devices.
The problem with these solutions
that policies directed at alleviating the symptoms of a problem usually fail because they trigger compensating feedbacks, feedbacks that undercut the intended effects of the policy.

Martin Treiber has an excellent Java based simulation tool on visualization of traffic congestion. Definitely worth a watch.. The link is HERE

Any suggestions on improving ? Anyone has a positive experience in their cities or is commute equally bad for everyone out there ?


Anonymous PG said...

Nice blog Ujval. Yes, I agree that traffic problems are very complex and have a zillion factors! They definitely cannot be solved by one change in the system. For eg- bringing metro into the Dulles corridor is not going to help if major connecting points are not developed for mass transit. Adopting methods you have mentioned (HOV, toll etc)will only make existing problems take another form, will not eliminate them. They are only making us more working around them.
Eliminating traffic problems will require a lifestyle change combined with a huge change in urban planning & city layout.

7/22/2005 1:13 PM  

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