The design of Missouri highway bridges has changed many times over the years. It is usually possible to estimate the age of a bridge by the guard rail construction.

The first two examples are of the original design used in the 1920s. These are bridges on SR N three miles west of Green City, built when MO 6 originally followed this route.

The western bridge over Little Yellow Creek, built in 1922.

The eastern bridge over Yellow Creek, built in 1923.

The following example is also of the original design used in the 1920s and 1930s. This particular bridge is on MO 5 over Elmwood Creek one mile north of Milan. It was built in 1929, and is on the list for replacement by 2012.

Next is an example of the design used in the 1940s, when construction of the secondary system was underway. This example is on SR E over West Fork Locust Creek, 9 miles northwest of Milan, built in 1940. The bridge is marked one lane with a 15mph speed limit for vehicles over 16 tons.

This is the same bridge from below. Notice the piers: vertical members of steel, horizontal ones of wood.

Another method used at the time was pony truss construction. This example is on SR T over East Fork Locust Creek, 9 miles south of Milan, built in 1949. It is open as 2 lanes (for a while was one-lane) and has a speed limit of 15mph for single-unit trucks over 12 tons and multiple-unit trucks over 17 tons.

This is the same bridge viewed from below.

Here is an example of the construction used in the 1950s. This is on MO 6 over the Chariton River seven miles west of Kirksville, built in 1953. It replaced an earlier bridge when the highway was rebuilt for flood effect reduction. In 1975, MO 6 was realigned between Novinger and Kirksville; the west end of the new road tied into the east end of this bridge.

Here is the same bridge viewed from below.

Another example is on SR PP over the East Fork Medicine Creek near Osgood, built in 1955. It used the pony truss method. Due to width restrictions, it has been posted as a single-lane bridge since at least the late 1960s. It also carries a speed limit of 15mph for single-unit trucks over 11 tons and multiple-unit trucks over 15 tons.

Here is the same bridge viewed from below. Notice that the piers were actually wide enough to accommodate a wider deck.

Here is an example of the construction used in the 1960s. This bridge is on SR OO over MO 5 / MO 6 in Milan, built in 1962 to accommodate construction of a bypass route that currently carries MO 5 and MO 6 (thus creating Business 5).

Here is an example of the construction method used in the late 1960s and 1970s. This is on SR E over Locust Creek, 2 miles west of Milan, built in 1969 as part of a road realignment project opened to traffic in 1970.

Here is an example of the construction used starting in the mid-1980s. This is on MO 6 over Locust Creek, 8 miles southwest of Milan, built in 1985 as a replacement of the original bridge.

A later example of the current method is on MO 6 over East Fork Locust Creek, 1/2 mile east of Milan. This bridge replaced an older pony truss bridge in 1996.

At times MoDOT retrofits newer guard rails onto older bridges to either accommodate wider traffic lanes or to replace dangerously worn concrete. In these examples, the new rails are metal, and built in the 1990s.

This bridge is on MO 6 over a branch of Yellow Creek, five miles east of Milan. The original bridge was built in 1956 as part of a road realignment project between Milan and Green City.

This bridge is on MO 6 over West Fork Locust Creek, 11 miles west of Milan. The original bridge was built in 1932.

Go here to see some historic county road bridge photos.