Shared Mobility

How Many Cars Do We Need for Carsharing to Eliminate Traffic Jams?

Traffic jams cause 80 percent of additional pollution and three to four times longer travel times in cities. With too many cars carrying just a driver and several empty seats, streets come to a standstill daily. If more drivers shared their cars we could easily alleviate these problems.

Most of us use our cars inefficiently, using just one seat for the driver and leaving more than four others empty. It is estimated that for every five cars that drive through Madrid, four carry just a single driver, while the fifth car only carries two people.

With this usage pattern, we use many more cars than are necessary, squandering fuel and polluting the air we breathe.

Everything gets complicated by the daily evolution of traffic.

Everyone is subject to different schedules and the demand for seats fluctuates greatly throughout the day. As indicated by the graph below, the supply of both vehicles and public transit is consistent over the course of the day (blue line of the graph). However, most transportation options are unable to handle peak demand during rush hour when available transport doesn’t seem to be sufficient.

Urban Traffic

To get a fuller picture of the problem, consider the following: If there are one million cars on the road in Madrid every day, we are wasting four million valuable empty seats. This is more empty seats than there are people traveling by metro and bus. Additionally, most of these seats are vacant at peak times, when we suffer most from transportation problems.

Could we take advantage of our empty seats to have fewer problems and live better?

Let’s study in detail why traffic jams occur, and their pernicious effects. In the following graph, the black line represents how the number of vehicles on the road over time (the actual traffic) evolves. On the left side we have growing but moving traffic, which is able to satisfy the demand. Problems begin when the maximum capacity of the road is reached. This usually happens on Madrid’s M-30 at about 7:00am.

When the number of vehicles on the road exceeds its maximum capacity, the road becomes congested. Cars start braking and, suddenly, the capacity of the road to admit vehicles is drastically reduced.

Peak time

From that moment on, despite the fact that traffic continues to rise, the number of cars actually drops. We pass into a congested traffic situation, where the black line showing the real traffic decreases and drifts apart from the demand curve (the discontinuous blue line, which continues to grow); So that more and more of the demand is unmet. Cars have to wait to be able to enter the main routes or they have to take a detour looking for an alternative road.

The most notable and damaging effects of traffic jams are:
A sharp decrease in speed (eg. on Madrid’s M-30 the speed drops from 55 to 15 MPH), which makes travel times three to four times longer.
speed evolution
Each car caught in the jam emits 80 percent more pollution (According to a study from Dr. Treiber from the University of Dresden).
pollution evolution
Once we have congested traffic, the situation seems insurmountable. Despite the fact that the number of cars on the roads decreases, the bottleneck means that traffic remains at a standstill. The only way to solve this challenge is to decrease the number of cars entering the road in the first place to avoid a traffic jam occurring.

The only solution is to remove a sufficient number of vehicles to avoid the roads from becoming congested.

Imagine that we managed to convince a certain number of people to offer the empty seats in their cars to passengers on their way to work. Thus removing several vehicles (from one to four, according to the number of passengers they carry) from the road.

This behavior change, which efficiently takes advantage of the sharing of the empty seats of some cars, helps in two ways:
All the people who need to get to work do so in fewer vehicles, which means fewer cars on the road and less traffic.
people in fewer cars
The effective capacity of the roads increases (according to the green area of the following graph).
extra capacity
This also has two effects on pollution improvement:
– First of all, with less vehicles on the road, there is less pollution.
– Additionally, by removing the extra cars, we prevent the pollution caused by traffic jams, which accounts for 80 percent more pollution than regular traffic.
How many rides do we have to share to avoid the traffic jam?

To keep a road decongested, we have to remove the excess cars that cause the jam (the number of cars between the saturation level of the road and the peak of the demand).

To achieve this, it would suffice to share a relatively low number of rides. The fact that each carpool can offer up to four passenger seats, is a multiplier effect that works in our favor.

In the worst case (if each car shares only one passenger, and still keeps three empty seats), it would be necessary to share as many cars as the surplus demand over the capacity of the road.

However, in the best case (when each car fills all its empty seats and carries four passengers) the situation is as shown in the following graph, and the number of cars needed is four times lower.

The rest of the vehicles can continue with one driver, and both these vehicles and the shared cars will enjoy moving traffic, with the advantages of less pollution and lower travel times.

In conclusion, traffic jams in the city cause 80 percent more pollution and three or four times longer travel times. We can easily avoid them, by simply sharing the empty seats in some cars and carrying passengers going in our direction.


Please note that this article expresses the opinions of the author and does not reflect the views of Move Forward


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