Difficulty of comparison. — Must be between one bridge and several tunnels. —

Traffic in tunnels. — Ventilation of traffic tunnels. — Comparative agreeableness of passage by bridge and tunnel. — Comparative extents of climbing in bridges and tunnels. — Comparative costs. — Bridge versus Tunnel for the Proposed Hudson River Crossing at New York City. — Desirability of methods of handling traffic across the river. — Cost of operation. — Time expenditures. — Agreeableness. — Sanitation. — Ventilation. — Carbon monoxide danger. — Approach grades used in the comparison. — Tunnel estimates. — Variation of cost of tubes with diameter. — Comparative cost of a double-track tube and two single-track tubes. — Unit prices for bridge materials in place. — Limitation of cost by efficiency instead of aesthetics. — Cost diagram for bridges and tunnels. — Description of contrasted structures. — Fairness of comparison. — Cost of right-of-way and property damages. — Spiral approaches. — Table of total costs of bridges and tunnels. — Conclusion as to the comparative economics. — Temporary combination of bridge and tunnels. — Suggestion for further study of proposed crossing before committing the public to the present policy ...................... 53

Definition of terms. — Comparison generally a matter of expediency instead of one

of economics. — Using one through span over channel and making all other spans deck structures. — Advantages and disadvantages of the two types. — Approximate average figures of comparative costs. — Capitalization of various annual expenditures. — Power-cost capitalization unnecessary ............. 61

Difficulty in making comparison. — Variation in market prices an important factor.

— Must consider factors of maintenance and repairs. — Greater popularity of concrete structures among non-technical, uninitiated people. — Comparative limits of life. — Personal equation. — Science of reinforced-concrete-bridge designing not yet so highly developed as that of steel-bridge designing. — AEsthetics affects costs of reinforced-concrete bridges. — Live load affects the economic comparison. — Designing by inexperienced computers. — Variation in costs of excavation for foundations. — Quick method of settling this economic question. — Practicable limit of span-length for reinforced-concrete bridges. — Settlement of arches during construction. — Effect of height of crossing upon the economic comparison. — Unavoidable generality of treatment of subject and reasons therefor. — Impracticable to reach any fixed or reliable conclusion......... 64