To determine the appropriate extent to which Ferrovial SA should rely on hybrid bonds in its capital structure, we must analyze the company's financial health, the cost of capital environment, and the specific characteristics of hybrid instruments.

**1. Analysis of Ferrovial's Financial Position (2022 Data):**
*   **Equity Strength:** Total Equity is €6,354 million, with Equity Attributable to Owners of the Parent at €4,113 million. The company has a solid equity base.
*   **Debt Levels:** Noncurrent liabilities are €13,142 million, with significant borrowings (Noncurrent Portion of Other Noncurrent Borrowings: €10,776 million). The debt-to-equity ratio is relatively high, which is typical for infrastructure companies but suggests a need for careful capital management to maintain credit ratings.
*   **Profitability:** Profit Loss Attributable to Owners of Parent is €186 million. While positive, it is modest relative to the equity base (ROE ~4.5%). Hybrid bonds require coupon payments (often deferrable but accumulating or with step-ups), which can strain cash flows if profitability is volatile.
*   **Cash Flow:** Cash flows from operating activities are strong (€1,002 million), providing coverage for interest payments. However, investing activities show significant outflows for infrastructure projects (-€784 million) and acquisitions.

**2. Market Environment (2022 Data):**
*   **Rising Interest Rates:** The swap curves shifted dramatically from negative/low rates in 2020-2021 to positive, higher rates in 2022 (e.g., 10Y Swap Average rose from 0.053% in 2021 to 1.927% in 2022). This increases the cost of all debt, including hybrids.
*   **Credit Spreads:** The iBoxx EUR Non-Financial IG spread increased significantly (Average from 1.298% in 2021 to 2.295% in 2022). This indicates a tighter credit market and higher risk premiums.
*   **Hybrid Bond Cost:** Hybrid bonds typically yield higher than senior debt due to their subordinated nature and equity-like features. In a rising rate environment with widening spreads, issuing new hybrids becomes expensive.

**3. Role of Hybrid Bonds:**
*   **Equity Credit:** Rating agencies often grant partial equity credit (e.g., 50%) to hybrid bonds, which helps improve leverage ratios without diluting existing shareholders. This is beneficial for infrastructure firms with high capex needs.
*   **Flexibility:** Coupon deferral options provide flexibility during downturns.
*   **Risk:** They are permanent capital but can be called. If rates are high at issuance, calling them later to refinance at lower rates might not be possible if credit conditions deteriorate.

**4. Strategic Assessment:**
*   **Current Structure:** Ferrovial already has "Other Equity Securities" of €508 million (2023) / €507 million (2022), which likely includes perpetual subordinated bonds (hybrids). This represents a small fraction of total equity (~8-12%).
*   **Optimal Level:**
    *   **0%:** Too conservative. Hybrids offer tax-efficient capital and rating benefits. Given the high leverage, some hybrid usage is optimal to strengthen the balance sheet.
    *   **100%:** Impossible and impractical. Hybrids are expensive and cannot replace all senior debt or equity.
    *   **75%:** Too aggressive. Relying heavily on hybrids would significantly increase the weighted average cost of capital (WACC) due to the high yield demanded by investors for subordinated risk, especially in the 2022 high-rate environment. It also risks rating downgrades if the "equity credit" is not fully recognized or if coupons accumulate.
    *   **50%:** Still quite high for a company with modest profitability margins. While infrastructure firms use hybrids, a 50% reliance on *total capital structure* (Debt + Equity) being hybrids is excessive. Even as a percentage of *debt*, 50% is high. As a percentage of *total capital*, it's very high.
    *   **25%:** This is a balanced approach. It allows Ferrovial to benefit from the equity credit provided by rating agencies, thereby optimizing its leverage ratios and potentially maintaining a higher credit rating than pure debt would allow. It provides a buffer for financial flexibility without incurring the prohibitive costs associated with a larger hybrid issuance in a high-interest-rate environment. It aligns with the existing small portion of equity securities and allows for gradual optimization.

Given the rising cost of capital in 2022 and the need to balance leverage optimization with cost efficiency, a moderate reliance on hybrids is best. 25% represents a strategic tier that enhances capital structure efficiency without overburdening the income statement with high coupon costs.

<answer>25%</answer>