Recently, the The idea of congestion charging in Paris was handed up to date . The concept, already tested among others in London and Singapore, is to pay a small sum to all drivers entering a zone determined. This policy is typically very popular among economists and very unpopular among non-economists. Why?
Once is not custom, we'll do some theory. We will try to imagine a simplified world in which a large number of people simultaneously want to go from point A to point B. To make this journey, they have a choice between taking their cars or take public transit. We will make several assumptions to simplify the analysis. First, we assume that the only concerns of individuals is to minimize travel time and the cost of one mode or another is the same *. Second, we assume that public transport has a great capacity and reliability does not depend on the number of passengers. Simply put, we say that the journey by public transport always takes 1 hour, regardless of the number of passengers (we return to this assumption earlier). Third, we will say that the travel time by car depends on the number of people who chose to take their car at the same time (because of congestion).
Let's think about how people will behave. If the drive takes less than an hour, then people who used to take public transport will change their mind and choose to take their car, increasing congestion and also travel time by car. If the drive takes over an hour, then some people taking their car will change its mind and take public transportation instead. Whatever you do, ultimately, the travel time by car is, in this simplified world, equal to the travel time by public transport because any other configuration will push some people to change their minds. This is called a "balance" in our jargon.
Why is this a problem? If I'm in a hurry, it bothers me to have to be forced to have an hour of transport, whatever means of transport. Maybe I'll be ready to pay a small sum to save time. Let us for example, that I would find beneficial to pay 5 € to be able to make the journey in half an hour. But in this configuration, I can not.
But what happens if the authorities decide to set up a toll? For example, they may decide to pay 5 € to all those who travel by car. So, the travel time by car will no longer be equal to the long travel time by public transport. Some people will choose to leave their cars at home and take public transport, which will result directly reduce congestion. In the end what would there be? Each individual may choose between taking public transport and make his way in an hour or take his car, and pay 5 € to its course in 40 minutes.
Ultimately, the situation has improved for everyone . Individuals "slow", not wishing to pay for twenty minutes will make the transport as before. Those, however, prefer to pay a little more expensive to gain a bit of time will be happy to do so. With the tax on road use, it has improved its use of resources in our fictional world **.
What can we learn from all this? First, we see that the fundamental assumption of the model is that there is an alternative to the car, that is to say, Transit capacity and quality. However, in Paris, the transport is already saturated at peak hours. Anyone who is already mounted on the RER A in the morning was realizing it. How does this change he's analysis?
If public transport themselves are congested, a congestion charge is unlikely to change behavior. Result, this tax will be an extra weight in the budget of individuals without real counterpart. To be effective, requires that individuals have choice in their modes of transportation. If indeed there is a bus line unsaturated allows me to make the journey that I am currently in the car, then a tax will change behavior and individuals improve the efficiency of the system. The problem also arises for Areas served by public transport. If I live in a town lost in the suburbs (because I can not afford to live closer), I have no other choice but to take the car and the toll will be ineffective.
So if you want the system works in Paris it is imperative increase the supply of transport. The London authorities had committed themselves at the time of the introduction of congestion charging, reusing the revenue collected to improve the quality of public transport common, which proves that they had grasped this part of the problem.
Some mention the environmental aspects. Obviously if the congestion charge has a significant effect on behavior, emissions of greenhouse gas emissions will decrease. But that should not be the main objective of this policy. Why? Because 1) there are other more appropriate policies to fight against pollution as the carbon tax and 2) contrary to popular belief, pollution per capita in cities is much lower than pollution per capita in rural areas. If cities are more polluted, it's simply because there are many more people, but on average, each inhabitant pollute less (partly because fewer long journeys and there is greater energy efficiency in homes and transport).
* We could easily generalize the assumption that individuals have preferences for one or the other modes of transport based on comfort and each transportation mode has a different monetary cost, but it would complicate the model without changing the conclusions.
** Those who have done a little economy here will recognize the concepts of externality and Pigouvian tax. When a person takes his car, it imposes an external cost to all others because it increases traffic. By charging the cost to the individual through a tax, it improves the efficiency of the market because it can lead individuals to internalize the externality.
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