To determine the optimal reliance on hybrid bonds for Ferrovial's capital structure, we must evaluate the cost and benefits of hybrids relative to pure debt and pure equity. 

1. **Cost of Capital**: Hybrid bonds carry a higher yield than standard senior debt because they are subordinated and may have interest-deferral features. Standard corporate debt usually provides a lower cost of capital.
2. **Tax Shield**: While hybrid bonds can sometimes offer tax-deductible interest, standard straight debt offers straightforward, universally recognized tax shields without the structural complexity or potential partial equity treatment by rating agencies that diminishes the tax benefit per unit of capitalization.
3. **Financial Distress**: Unlike equity, which permanently absorbs losses and requires no mandatory dividend payments, hybrids still carry coupon expectations (even if deferrable) and limit financial flexibility compared to common equity. 

In traditional capital structure optimization models (like those based on Modigliani-Miller with taxes and distress costs), hybrid bonds are generally strictly dominated by a well-calibrated mix of standard straight debt and common equity. Straight debt optimizes the tax shield at the lowest cost, while equity provides the ultimate buffer against financial distress. As a result, the mathematical optimum for a standard non-financial corporate's capital structure allocates 0% to hybrid bonds.

<answer>0%</answer>