To determine the extent to which Ferrovial should utilize hybrid bonds relative to S&P Global Ratings' 15% cap, we first need to calculate the company's total adjusted capital and the maximum allowable amount of hybrid bonds.

1. **Calculate Total Adjusted Capital:**
S&P defines total adjusted capital as Debt plus Equity. 
From the balance sheet data as of 2023-01-01:
- Total Equity = 6,354,000,000 EUR
- Total Financial Debt = Noncurrent Borrowings (10,776,000,000) + Current Borrowings (877,000,000) = 11,653,000,000 EUR
Total Adjusted Capital = Equity + Debt = 6,354,000,000 + 11,653,000,000 = 17,807,000,000 EUR

2. **Calculate the S&P Maximum Hybrid Cap:**
S&P limits the equity credit given to hybrids to a maximum of 15% of total adjusted capital.
Maximum Hybrids Allowed = 15% × 17,807,000,000 = 2,671,050,000 EUR

3. **Evaluate Current and Potential Hybrid Usage:**
We look for existing hybrid instruments. Ferrovial has "Perpetual Subordinated Bonds" listed within equity, which are classic hybrid bonds. The balance for this account on 2023-01-01 is derived from the movements:
2022 Beginning Balance (Other Equity Securities) = 507,000,000 EUR
Plus 2022 Movement = 1,000,000 EUR
2023 Ending Balance = 508,000,000 EUR.

Additionally, since the company's credit spreads (as indicated by the iShares Core Euro Corp Bond and sub-sen deltas) rose in 2022 to over 1.0% and are higher than previous years but remain at investment-grade levels, issuing additional hybrids is viable for optimizing the cost of capital while maintaining the ~50% (BBB+) equity credit S&P typically grants to such instruments at issuance. 

4. **Determine the Optimal Utilization Extent:**
The question asks to what extent the company *should* utilize hybrids *relative* to the 15% cap. 
Ferrovial's existing hybrids (508M) represent about 19% of the cap. To optimally manage its capital structure, leverage its strong infrastructure asset profile (which naturally supports high leverage in S&P's methodology), and minimize the cost of capital without risking downgrades or "zero equity credit" thresholds, a company of this profile typically targets a middle-ground utilization. A 50% extent of the cap means targeting roughly 7.5% of total adjusted capital in hybrids (approx. 1.3B EUR), which is standard for infrastructure-heavy entities looking to aggressively but safely manage leverage ratios without overshooting into risky territory. A 25% or lower extent would be overly conservative given their asset class, while 75-100% would aggressively push the limits of rating agency tolerance and risk full deduction of equity credit. Thus, 50% utilization relative to the cap is the most strategically sound extent.

<answer>50%</answer>