To determine the appropriate reliance on hybrid bonds for Ferrovial's capital structure, we must analyze the company's financial characteristics and the strategic role hybrid bonds play for such firms.

1.  **Business Model and Capital Needs**: Ferrovial is a major infrastructure and construction company. Its balance sheet shows massive investments in infrastructure projects (€13.67 billion in 2022), which are characterized by extremely long payback periods and high capital intensity. This creates a natural asset-liability mismatch if funded purely with short-term debt or even standard long-term debt that has defined maturities. 

2.  **Current Capital Structure**: As of the end of 2022, Ferrovial’s total equity was €6.35 billion, and total noncurrent liabilities were €13.14 billion. Long-term borrowings amounted to €10.78 billion, highlighting a heavy reliance on traditional long-term debt to fund its infrastructure assets. Furthermore, the company already utilizes perpetual subordinated bonds (a form of hybrid capital) as seen in the equity movements, indicating familiarity with the instrument. 

3.  **The Role of Hybrid Bonds**: For infrastructure entities like Ferrovial, hybrid bonds (perpetual subordinated bonds) are highly attractive. Under IFRS, these instruments are often classified as equity (as seen with Ferrovial's perpetual subordinated bonds), which supports key credit rating metrics and leverage ratios without diluting shareholder ownership. Their perpetual nature perfectly matches the ultra-long duration of infrastructure assets, mitigating refinancing risk. They also allow the company to return capital to shareholders via share buybacks while maintaining a strong balance sheet.

4.  **Extent of Reliance**: While hybrid bonds are an excellent strategic tool for an infrastructure firm, they come with significantly higher coupon rates compared to senior secured debt. Relying on them for 50% or more of the capital structure would be excessively costly and inefficient. Traditional debt, bank loans, and project finance remain the most cost-effective funding sources. However, a nominal or minor reliance (such as 0%) would neglect the strategic benefits of equity-credit and duration-matching that hybrids provide. Thus, a modest allocation—roughly up to 25% of the total capital structure—is optimal for infrastructure companies to balance rating preservation, cost of capital, and duration matching without overpaying for capital.

Given the massive scale of Ferrovial's debt needs and the cost premium of hybrids, a 25% reliance represents the standard prudent ceiling for such instruments in an infrastructure firm's capital structure.

<answer>25%</answer>