As civilization grew centuries ago, man created land transportation – but he ran into obstacles in the form of waterways. The solution to that problem: bridges. As shipping and boating grew, man ran into obstacles again – this time in the form of the bridges themselves.

Thus the drawbridge was created.

The hor signal to open the bridge is one long blast followed by one short blast. The tender ackowledges the same way to let you through.

Drawbridges allow the free flow of vehicle traffic if no boats are trying to pass, but they also allow vessels to continue cruising with only a brief interruption for the land traffic. While all drawbridges are the same in the sense that they have a section of moveable deck area, the way in which that section opens is where the differences lie. Some of them are quite ingenious.

The most common methods are the swing bridge, the bascule bridge, and the lift bridge. With a swing bridge, the moveable span pivots horizontally at its midpoint. A bascule bridge contains moveable spans – usually two – that pivot vertically, like a pair of upward-opening doors. And a lift bridge features a moveable span that raises straight up between two towers to a height that allows clearance for any anticipated ship traffic.

While the whole system is pretty simple in the abstract, there are issues that arise when you are traveling. For example, between land traffic and water traffic, who has the right of way?

THE UNITED STATES COAST PILOT

Rules governing drawbridges are found in the United States Coast Pilot, a series of books that cover a variety of key information for navigators of coastal and intracoastal waters, as well as the Great Lakes. Topics include channel descriptions, bridge and cable clearances, and tide and water levels. The books are issued in nine volumes and contain supplemental information that is difficult to portray on a nautical chart. While the Coast Pilot doesn't contain formal or legal right-of-way rules, in general terms, it explains that boats usually have right of way over vehicles, and trains have right of way over boats.

This lift bridge does what its name states - lifts. that way, sailboats and other tall vessels can pass.

Let's focus on trains for a moment. The Coast Pilot points out that train traffic is controlled by a system of blocks. Once a train is in a block, any drawbridge that is part of that block is locked in the closed position until the train clears the block. The good news for boaters is that many drawbridges on infrequently used rail lines are normally kept in the open position. They're only closed when a train approaches.

The situation with vehicle traffic is more complicated. Each volume of the Coast Pilot focuses on a different region of the country, such as Alaska and the Great Lakes and their connecting waterways. While every volume discusses drawbridges – specifically in section 117 – that section actually has two subparts. One explains the general rules governing all drawbridges, and the other lists exceptions to the rules for specific bridges in the area covered by that volume. These exceptions usually consist of times that the drawbridge isn't required to open, or specific times, such as on the hour, when the bridge will open for boat traffic. If you'll be cruising outside of your local area, it's wise to have the appropriate Coast Pilots aboard, not only to learn about rules for specific bridges, but also for the wealth of other information that they contain.

HOW TO REQUEST AN OPENING

So, how does a drawbridge operator – also called a drawbridge tender – know that a boat wants to go through? The captain needs to signal his or her intention in some way. There are a number of ways to do this, but there are also rules governing what not to do.

For example, a boat should not signal for an opening "if the vertical clearance is sufficient to allow the vessel, after all lowerable nonstructural vessel appurtenances that are not essential to navigation have been lowered, to safely pass under the drawbridge in the closed position." That means don't be lazy; if you can get under the bridge by lowering antennas and such, do so. The chart will tell you the clearance under the closed bridge at high water, and some bridges have a clearance gauge that indicates the measurement at the current stage of the tide.

If more than one boat approaches a drawbridge, each needs to signal for it to open.

Assuming that we are dealing with a bridge that opens on demand, there are several ways to request an opening. Probably the simplest and most convenient is simply to call the bridge on the radio. Many drawbridges have a sign listing the calling and working frequencies monitored by the tender. Talking directly to him or her has the advantage of providing immediate feedback.

The next most convenient way to signal a drawbridge is to use a sound signal, such as your boat's horn. The official signal to request an opening is a prolonged blast followed by a short blast no more than three seconds later. A prolonged blast is defined as lasting four to six seconds, and a short blast lasts about one second. The drawtender is supposed to respond within 30 seconds. You're required to repeat the request until you get a response. If the bridge can be opened, the drawtender will answer you with a similar signal; if, for some reason, the bridge cannot be opened immediately, the drawtender will respond with five short blasts.

The Coast Pilot also lists visual signals. A boat can request an opening by raising and lowering a white flag or a white, amber, or green light in a vertical direction. If the tender can open the bridge, he or she will respond similarly, or with a fixed or flashing white, amber, or green light. If he or she can't open the bridge right away, he or she will respond with a red flag, a red light swung back and forth horizontally or a fixed flashing red light.

Regardless of which signal you use and get in return, once the bridge is opened, proceed through as promptly as possible. If you approach a bridge that is already open, you should still give the opening signal. If there is no acknowledgement within 30 seconds, you're permitted to proceed, with caution, through it.

Now, the scenarios just given assume that you're the only boat in the area. The Coast Pilot thankfully does cover a number of situations where multiple vessels are involved. If two or more vessels are approaching a bridge at the same time, perhaps from opposite sides, they each need to signal and get a response. However, boats accumulating to transit beneath the bridge during a scheduled opening don't need to signal individually. And in a situation where there are two or more drawbridges close together, there must be a request to and acknowledgement from each bridge in turn.

Obviously, the rules can be suspended for emergency situations. If an emergency vehicle is responding to an urgent situation, the drawtender must make every effort to have the bridge closed by the time the vehicle arrives there. Similarly, if possible, the draw should be opened for official vessels used for public safety, vessels in distress (or those assisting them), and vessels seeking shelter from severe weather – even if it conflicts with the operating schedule of the bridge. Drawbridges do not need to open during periods of natural disasters or civil disorders.

The needs of both land vehicles and boats sometimes conflict at their points of intersection. These rules do a good job of treating the needs of each fairly, though it may require some patience on each side.