The term high-level is applied to a structure having all its spans fixed and its deck at a considerable elevation above high-water level; but the term low-level is generally interpreted as applying to structures having one or more movable spans, although, strictly speaking, a bridge over a non-navigable stream may be a low-level one without having any movable span.

The comparison between a proposed high-level and a proposed low-level crossing for any stream is generally more dependent upon the condition of expediency than it is upon that of economy. The convenience of the passengers is often the criterion that will settle the question; but sometimes convenience has to be ignored because of the paramount condition of first cost. Again, the comparison will depend largely upon how much higher the high-level bridge must be than the low-level one, and upon the height and slope of the banks. The clear channel required will also be of some importance, the wider the channel the greater the advantage for the high-level bridge.

Sometimes in a long, high-level structure with only one channel span required, it is practicable to build all the others on grade and as deck spans, economizing on substructure and shortening the approaches, thus lowering materially the total cost of the high-level bridge.

The advantages and disadvantages of a low-level bridge over a high-level one are as follows:

Advantages

a. The first cost is almost always less.

b. The costs of maintenance and repairs are less.

c. The entrance and exit are closer to the river.

d. The climb is less, and, therefore, the total amount of power of
           all kinds required for the climb is less.

e. The time spent in crossing is less.

Disadvantages

f. Increased annual expense due to the several items of cost in
          connection with operating the movable span.

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