The rise of cars in the 20th century allowed for much faster travel in many cities, especially in developed countries. One could expect this increase in speed to be a time-saver, which would give households more work or free time. Studies directed in the 1970s by Jacob Zahavi disproved that theory. By analyzing travel time budgets in several cities over the world, Jacob Zahavi observed that households took advantage of the faster speeds not to reduce their travel time, but to go further and access more opportunities. This idea was formalized as Zahavi's conjecture. It states that cities are characterized by a constant travel time budget over time of around 1 hour. Several studies have backed up this conjecture in many other cities over the world. For instance, let us consider the Paris region. In 2001, the average time spent in transport was relatively stable and around 1 hour and 10 minutes, whatever the area of residence, from Paris to the most remote areas and whatever the motorization level of the household. Zahavi's conjecture must be understood as an average behavior observed for a sufficient number of individuals like a law of large numbers. At an individual level, travel time budgets vary widely depending on the individual's age, economic activity, etc. The increase in speed does not lead to time saving but to an increase in traveled distances. Let us look at Paris region again. In dense areas, traffic congestion, low speeds, and traveled distances remain moderate. But as we move further away from the center of the agglomeration, congestion decreases, speed increases, which leads to longer distances traveled per day. Why the increase in distance? Households take advantage of the speed to move further away and enjoy cheaper real estate or gain easier than before access to new opportunities, shopping centers, places to go out, etc. This leads us to the second conjecture of Zahavi which states that a household's average transport monetary budget, is also stable over time and around 15% of the city in question's average income. When cities develop economically, they use cars more often. An increase in income leads to an increase in transportation costs, which keeps the transportation burden around 15%. Although it was corroborated by other studies, the second Zahavi conjecture is not as solid as the first one. It must be understood from a global standpoint, in other words, at the metropolitan level. Within a metropolis, the increase in traveled distances leads to a strong increase in transport burdens as we get further away from the center. In Paris region, the average burden ranges from 8% for Paris inhabitants to 21% for households living the furthest away, in rural areas. The motorization rate also plays an important part as cars are more expensive than public transportation, as Zahavi noted. As a conclusion, the first Zahavi's conjecture is sometimes called "Zahavi's curse". Introducing more efficient transports leads to an increase in traveled distances and thus, to an increase in environmental nuisances, especially greenhouse gases. To avoid this race for more and more speed, some researchers advocate returning to slow cities, to allow for more sustainable mobility. This solution would be less expensive than another solution involving massive investments in greener technologies which would reduce emissions per kilometer traveled and offset the effects of distance but could be very expensive.