📈 economics

Airport pricing and capacity with non-aviation markets: single-till versus dual-till regulation

This study evaluates single-till versus dual-till airport regulations by demonstrating that the optimal choice for social welfare and the extent of runway capacity overinvestment depend critically on whether non-aviation profits are exogenous or endogenous and on the degree of complementarity between aviation and non-aviation demands.

Original authors: Ming Hsin Lin

Published 2026-07-20

📖 7 min read🧠 Deep dive

CC BY 4.0

Original authors: Ming Hsin Lin

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are running a massive, bustling train station. Your job isn't just to let people catch trains; you also run the snack bars, the parking garage, and the gift shops. In the world of economics, this is the story of airports. For decades, experts have debated a tricky question: How should the government regulate the prices at these stations? Should they look at the airport as one big business where the profits from the gift shop can help pay for the runway? Or should they treat the runway and the gift shop as two completely separate accounts, where the runway has to pay for itself using only ticket fees? This debate is known as "single-till" versus "dual-till" regulation. It matters because if the rules are wrong, airports might charge too much for tickets, build runways that are too small (or too big), or even lose money, forcing taxpayers to bail them out.

This paper, written by Ming Hsin Lin, dives deep into that debate using a mathematical model that acts like a giant simulation. The author asks: What happens when we consider that the things people buy at the airport (like parking or hotels) are often "complements" to flying? In other words, if you are flying, you probably need a place to park your car or a hotel room. The paper explores two different worlds. In the first world, the airport has no control over how many people buy these extra services; it's just a fixed number. In the second, more complex world, the airport can actually decide how many parking spots or hotel rooms to offer and what to charge for them. The study uses these scenarios to see which regulatory rule—single-till or dual-till—leads to better outcomes for everyone, including lower ticket prices and happier travelers.

The Great Airport Accounting Debate

Let's break down the two main characters in this story. First, there's Single-Till Regulation. Imagine the airport is a single piggy bank. All the money coming in—whether from landing fees, ticket taxes, or selling a bag of pretzels—goes into that one bank. The rule is that the total money in the bank must cover all the costs. If the pretzel stand makes a huge profit, it can help pay for the runway repairs. This means the airport might be able to charge less for landing a plane because the snack bar is helping out.

Then there's Dual-Till Regulation. Here, the airport has two separate piggy banks: one for "Aviation" (runways, planes, tickets) and one for "Non-Aviation" (shops, parking, hotels). The rule is strict: the Aviation bank must cover the Aviation costs on its own. The profits from the snack bar cannot be used to lower the ticket price. If the runway costs more than the tickets bring in, the airport must raise ticket prices, even if the gift shop is making a fortune.

The First Scenario: When the Airport Just Watches

In the first part of the study, the author imagines a situation where the airport can't really control the non-aviation side. Maybe the parking lots are full, or the shops are run by other companies, and the airport just collects a fee. In this "exogenous" world, the math shows a clear winner.

When the airport can use the profits from the shops to help the runways (Single-Till), it charges lower fees for landing planes. Because the fees are lower, more airlines want to fly, and more people get to travel. This leads to a higher "social welfare," which is a fancy way of saying "everyone is happier and better off."

However, under Dual-Till, the airport is forced to make the runway pay for itself. It has to charge higher fees to break even. This makes flying more expensive, fewer people fly, and the overall happiness of society drops. The study finds that as long as the non-aviation side is making money (like a busy parking lot), letting that money help the runway (Single-Till) is the better choice.

The Twist: When the Airport Plays Both Sides

The story gets much more interesting in the second scenario. Here, the airport is smart and active. It decides exactly how many parking spots to build and what price to charge for them. It anticipates how its choices will affect the ticket prices. This is the "endogenous" world.

In this scenario, the rules of the game change completely. The study finds that under Dual-Till regulation, the airport has a strange incentive. Since it can't use shop profits to help the runway, it tries to make the runway look as profitable as possible. To do this, it actually sets the price of non-aviation services (like parking) lower than it should be—sometimes even losing money on them! Why? Because cheap parking makes people want to fly more, and the airport thinks, "If I lose a little on parking, I'll make a huge profit on the runway tickets."

Under Single-Till, the airport does the opposite. It sets the price of non-aviation services higher because it knows the runway is already being subsidized by the total profit.

So, who wins now? It depends on how closely linked flying and the extra services are.

If the link is strong (people really need parking when they fly): The Dual-Till approach actually wins. The airport's strategy of "losing money on parking to boost flying" turns out to be a clever way to maximize overall happiness. The study suggests that when the demand for parking and flying are tightly connected, the Dual-Till airport ends up with a better outcome for society than the Single-Till airport.
If the link is weak (people don't care much about parking): The Single-Till approach wins again. The clever trick of Dual-Till doesn't work as well, and the old rule of "let the shops help the runway" is still the best path.

The Over-Engineering Problem

Finally, the paper looks at how much runway the airports decide to build. In both scenarios, the study finds that airports tend to overinvest. They build runways that are bigger than what is truly necessary for the most efficient outcome. It's like building a highway with six lanes when four would do, just because the math of the regulation makes it look cheaper to build more than it actually is.

However, the amount of overbuilding changes based on the rules.

When the link between flying and shopping is strong, Dual-Till airports overbuild less than Single-Till airports.
When the link is weak, Dual-Till airports overbuild more.

The Bottom Line

This paper doesn't just say "Single-Till is good" or "Dual-Till is bad." It shows that the answer depends on how the airport behaves and how connected the different parts of the airport are.

If the airport is just a passive collector of fees, Single-Till is the clear winner because it allows cross-subsidies to lower ticket prices. But if the airport is an active manager that can set prices for shops and parking, Dual-Till can sometimes be better, but only if the connection between flying and those extra services is very strong. The study demonstrates that while airport congestion is often cited as a key factor, it may not be a decisive factor in determining the best regulatory approach. Instead, the way airports manage their non-flying businesses and how tightly those businesses are tied to flying are the real keys to getting the regulation right.

Technical Summary: Airport Pricing and Capacity with Non-Aviation Markets: Single-Till versus Dual-Till Regulation

Problem Statement
Public airports often face a financial dilemma where socially optimal pricing (first-best) results in operating deficits, necessitating government subsidies. To ensure self-financing, regulators often impose cost-recovery constraints. The central policy debate concerns whether to regulate airport pricing based on the "single-till" approach (considering total profits from both aviation and non-aviation activities) or the "dual-till" approach (regulating only aviation profits). While existing literature has examined this issue, previous studies often rely on restrictive assumptions, such as ignoring the vertical structure between airports and airlines, treating non-aviation market decisions as exogenous, or failing to explicitly derive the degree of budget constraints (Lagrangian multipliers) under different regimes. This study addresses these gaps by investigating airport pricing and runway capacity decisions in the presence of non-aviation markets, specifically evaluating the relative performance of single-till versus dual-till regulations.

Methodology
The paper employs a unified theoretical framework combining general functional approaches and specified functional forms.

Model Structure: The model features a congested airport served by duopoly airlines (Cournot competition) and a monopolistic airport providing non-aviation services (e.g., parking, hotels) that are strict complements to air travel. Passengers make simultaneous purchase decisions for both services.
Regulatory Framework: The analysis formulates the optimization problem for budget-constrained (BC) public airports. The cost-recovery constraint is parameterized by $\omega$ , where $\omega=1$ represents single-till regulation (total profit $\Pi_A + \Pi_N = 0$ ) and $\omega=0$ represents dual-till regulation (aviation profit $\Pi_A = 0$ ).
Scenarios: The study analyzes two distinct scenarios regarding non-aviation market decisions:
1. Exogenous: Non-aviation supply ( $X$ ) is fixed.
2. Endogenous: The airport optimally chooses non-aviation supply ( $X$ ) anticipating the per-passenger charge.
Analytical Approach: The authors derive first-order conditions (FOCs) for welfare maximization and profit maximization. Crucially, unlike prior studies that treat Lagrangian multipliers as parameters or only identify their signs, this paper explicitly derives the equilibrium values of these multipliers ( $\lambda$ ) to compare the "degree of budget constraint" across regimes.

Key Contributions

Integration of Decisive Factors: The study explicitly integrates four factors often treated separately or omitted in previous literature: (i) the vertical structure and runway congestion, (ii) demand complementarity between aviation and non-aviation services, (iii) endogenous decisions in non-aviation markets, and (iv) the differences in budget constraint degrees under different regulatory regimes.
Explicit Derivation of Multipliers: The paper is the first to derive actual equilibrium values for Lagrangian multipliers under single-till and dual-till regulations, enabling a direct comparison of social welfare and capacity investment efficiency that previous differential techniques could not achieve.
Unified Model: It employs a unified model to establish that optimal capacity investment rules share the same functional form across regulatory regimes, while simultaneously quantifying the differences in investment inefficiency (overinvestment).

Key Results

Scenario 1: Exogenous Non-Aviation Decisions
- Pricing: Single-till regulated airports set lower per-passenger charges than dual-till regulated airports. This is because single-till airports can use positive non-aviation profits to cross-subsidize aviation deficits, resulting in a lower degree of budget constraint ( $\lambda_1 < \lambda_0$ ).
- Welfare: Consequently, single-till regulation leads to greater aviation market output and higher social welfare compared to dual-till regulation.
Scenario 2: Endogenous Non-Aviation Decisions
- Pricing and Profit: A counter-intuitive result emerges: dual-till regulated airports set non-aviation prices below marginal cost, generating negative profits in the non-aviation market. Conversely, single-till regulated airports set prices above marginal cost, generating positive non-aviation profits. This occurs regardless of the degree of demand complementarity.
- Welfare: The welfare ranking depends on the degree of demand complementarity ( $h$ $h$ ).
  - When complementarity is high, dual-till regulation achieves higher social welfare than single-till. The negative non-aviation profit under dual-till acts as an optimal solution that, combined with strong complementarity, boosts overall welfare.
  - When complementarity is low, single-till regulation yields higher welfare.
- Capacity Investment: Both regimes lead to overinvestment in runway capacity. However, the magnitude of overinvestment under dual-till is smaller than under single-till when complementarity is high, and larger when complementarity is low.

Significance and Claims
The paper claims to provide "clear-cut results" that offer important policy implications for regulatory authorities. It challenges the prevailing view that airport congestion is the decisive factor in choosing between regulatory regimes. Instead, the study demonstrates that the endogeneity of non-aviation market decisions and the degree of demand complementarity are the critical determinants.

Specifically, the paper refutes the assertion by Kidokoro and Zhang (2022) that the sign of non-aviation profit is the sole crucial factor. This study argues that whether non-aviation decisions are endogenous or exogenous, and the strength of demand complementarity, fundamentally alter the superiority of a regulatory regime. The findings suggest that under high complementarity, the "unregulated" negative profit generation under dual-till can be welfare-enhancing, a nuance missed by models assuming exogenous non-aviation decisions. Furthermore, the analysis of capacity investment provides the first direct comparison of overinvestment inefficiencies between the two regimes, showing that the choice of regulation significantly impacts the efficiency of infrastructure investment.

The Great Airport Accounting Debate

The First Scenario: When the Airport Just Watches

The Twist: When the Airport Plays Both Sides

The Over-Engineering Problem

The Bottom Line

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