Transport planning requires anticipating how people travel today and how they will travel in the future. Without this knowledge, it would be impossible to design infrastructure, dimension public transport services, or evaluate mobility policies.

Among the different approaches available, the four-step transport model has become a reference methodology worldwide. It was first developed in the United States in the 1950s, during the era of motorway expansion, and has since been applied in cities across the globe. Its success lies in its structured way of representing mobility and its solid foundation for quantitative analysis.

This article explains how the model works, stage by stage, and discusses its limitations and future perspectives.

The first stage answers the question: how many trips are produced?

It estimates the number of trips generated and attracted in each zone of the study area. The calculation depends on:

• resident population,
• household income,
• car ownership rate,
• employment opportunities,
• the presence of schools, universities, health facilities, retail, and leisure activities.

• Regression models: link the number of trips to socioeconomic variables.
• Category analysis: divides the population into groups (age, income, occupation) with specific trip rates.

Once trips are generated, the next question is: where are they going?

This stage produces the origin–destination (O/D) matrix, which shows how many trips are made between each pair of zones.

• Gravity model: inspired by Newton’s law, trip flows increase with the size of zones (population, jobs) and decrease with travel time or distance.
• Opportunities models: consider the accessibility to opportunities along the way (e.g., available jobs or services located between origin and destination).

The third question is: which mode of transport do people choose?

This stage is critical, as it explains the division of trips between private car, public transport, walking, cycling, or other modes.

Traditionally, this decision is represented by the Generalised Cost (GC) equation, which converts different factors into a common unit: money.

• Out-of-pocket costs: fares, fuel, parking charges, tolls.
• Time costs: in-vehicle time, waiting, walking, transfers.

• Multinomial logit models (MNL): the most common, assigning probabilities to each mode depending on GC.
• Nested logit and probit models: used when modes are correlated (e.g., different public transport services).

The last question is: which routes do people take?

This stage allocates trips to the road network or public transport system, considering congestion and user behaviour.

• Wardrop’s equilibrium: each user chooses the route that minimises their own cost. The system reaches equilibrium when no user can improve their journey by switching routes.
• Stochastic assignment: introduces randomness to reflect that users may not perceive travel times perfectly.

The four-step model is widely used in:

• Mobility Plans.
• Road and rail infrastructure projects.
• Demand forecasts for new public transport services.
• Policy evaluation: congestion charging, low emission zones, parking pricing.
• Environmental and socio-economic impact studies.

Despite its robustness, the four-step model has one major limitation: it does not directly capture the user experience.

• service reliability,
• comfort and vehicle crowding,
• perceived safety,
• ease of transfers,
• environmental conditions (weather, urban design),

At Sanvi Consulting, we have worked to go beyond this limitation. Through user surveys and statistical modelling, we have translated these perceptions into quantifiable variables within the GC equation—either as equivalent time penalties or as latent variables with statistical weight.

This approach produces a more realistic model, one that explains not just how much a trip costs, but how it is perceived. That understanding is essential to anticipate what may motivate a person to shift from private car use to public transport or walking, and to design the right policies to support that shift.

The four-step model remains the reference framework in transport planning. Its clear structure makes it a powerful tool to simulate and forecast travel patterns.

However, the future of mobility modelling requires complementing it with approaches that account for the user experience. Only then can we design transport systems that are not only efficient but also attractive and aligned with people’s real needs.

At Sanvi Consulting, we believe that modelling must evolve from measuring euros and minutes to capturing the quality of journeys, enabling policies that genuinely foster sustainable mobility.