903.3 Ground-Mounted Sign Supports

903.3.1 Ground-Mounted Sign Installation

Forms

Signpost Selection Guide

Printable Signpost Selection Guide for use in the field

Guidance. Signs should be ground-mounted whenever possible unless mounting overhead is justified or required.

Standard. If signs are placed on existing supports, they shall meet other placement criteria contained in this article.

Utility and light poles shall not be used to mount signs as they are either not the property and maintenance responsibility of MoDOT or are not designed to carry the additional wind loading a sign adds to the structure.

Option. In areas with space restrictions, available sign truss columns, signal poles, bridge columns, or other significant MoDOT structures, excluding roadway lighting structures, may be used to mount flat sheet aluminum signs.

903.3.2 Lateral Offset (MUTCD Section 2A.19)

Support. The minimum lateral offset is intended to keep trucks and cars that use the shoulders from striking the signs or supports, while keeping signs and object markers close enough to the roadway to be effective.

Standard. Ground-mounted sign supports shall be crashworthy and meet the latest federal crash standards at the time of installation.

Guidance. The provisions below should be applied unless specifically stated otherwise in the EPG for a particular sign or object marker. For ground-mounted signs, the minimum lateral offset should be 12 ft. from the edge of the travelway. If an existing shoulder is wider than 6 ft., the minimum lateral offset for ground-mounted signs should be 6 ft. from the edge of the shoulder or front face of curb. See Figures 903.2.18.1 and 903.2.18.2 which illustrate typical examples of the lateral offset requirements contained in this portion of the article.

Maximum offset will depend on roadway geometrics, profiles, and cross-sections, which all affect the visibility of the sign. Signs are generally to be placed no more than 15 ft. from the edge of shoulder.

Ground-mounted signs placed in a gore only requires a minimum of 2 ft. lateral offset from edges of shoulder, face of barrier walls or guard rail.

For divisional and channelizing islands, a 2 ft. lateral offset should be maintained between the edge of sign and the front face of curb. For islands with restricted width the sign should not extend beyond the curb face.

Option. Deviation from the standard lateral offset may be used if a signs effectiveness and visibility are maintained to account for variations in roadside features. For example, to avoid placing signposts in the flow line of a ditch, avoiding drainage structures, pull boxes or sidewalks.

Option. Lesser lateral offsets may be used in business, commercial or residential areas where limited space is available to place signs due to limited right of way, sidewalks or other restrictions which keep the sign from being installed at the correct offset. In these cases, the edge of the sign may be placed up to, but not beyond the face of the curb making every effort to maximize the offset with the space available.

Guidance. Ground-mounted sign supports should not intrude into the usable width of a sidewalk or other pedestrian facility.

903.3.3 Mounting Height (MUTCD Section 2A.18)

Support. Installing signs at the proper mounting height is critical not only for the sign to be seen and function, but also to the functionality of the breakaway design for each type of signpost. Mounting heights are typically listed as “nominal” as it is critical to keep the signs a minimum distance above the ground, however, signs installed excessively high also have issues, such as:

• Place signs outside of the drivers line of sight
• Place signs outside the effective range of vehicle headlights at night making signs less visible
• Signs mounted too tall are more susceptible to wind resulting in signs being pushed out of plumb or being blown over

903.3.3.1 Mounting Height – U-Channel, Wood, Perforated Square Steel Tube (PSST) and Pipe Posts (MUTCD Section 2A.18)

Support. There are typically two mounting heights for signs on u-channel, wood, PSST and pipe posts, 5 feet and 7 feet. Traditionally, the 5-foot mounting height has been applied to “rural” areas and the 7-foot mounting high applied to “urban” areas or within incorporated city limits. However, the term “urban” has more to do with the conditions the signs are being installed within and less about being located within an incorporated city limit. The purpose of the 7-foot mounting height is to provide clearance for passing bicycle and pedestrian traffic, making the sign more visible over parked vehicles along the roadway and permits improved sight distance to drivers permitting them to see below the sign.

The 7-foot mounting height also applies to all freeway and expressway applications to enhance long distance visibility.

Standard. Standard Plans 903 shall be referenced for specific installation and mounting height details. The details in Standard Plans 903 and EPG 903.3 Ground-Mounted Sign Supports shall apply to all signs unless specifically stated otherwise for a specific sign or object marker elsewhere in the EPG.

The minimum mounting height of a sign shall be measured vertically from the bottom of the sign to the elevation of the near edge of the pavement. Minimum sign mounting heights shall be as follows:

• Sign located in rural areas – 5 feet,
• Sign located in urban areas – 7 feet,
• Signs located on freeways and expressways – 7 feet.

The length of post measured from the bottom of the sign to the ground shall also be a minimum of 5 feet. If the length of any post within a sign assembly measures less than 5 feet from the bottom of the sign to the ground, the minimum sign mounting height shall be increased to achieve the minimum 5-foot post length.

Option. Signs may be installed at 5 feet within the boundaries of incorporated city limits if the all following conditions apply:

• The sign is located outside of business, commercial or residential areas where there are no high densities of entrances and cross street intersections
• There is no on street parking
• There are no sidewalks with bicycle or pedestrian traffic

If a secondary sign is mounted below the primary sign on the same signpost(s), the mounting height for the assembly, measured from the near edge of the pavement to the bottom of the secondary sign, may be 1 foot lower than the minimums listed above.

In cases where signs are located on steep back slopes, it may be advantageous to relocate the sign forward or back from the proposed location to an area that is flatter to avoid a sign being mounted too high.

Guidance. Signs located outside of incorporated city limits that are located in areas having characteristics of an urban area, such as around businesses, heavy residential areas, areas with on street parking and areas with sidewalks which support bicycle and pedestrian traffic, should be installed at 7 feet.

Route marking assemblies consisting of a route sign and auxiliary signs should be treated as a single sign for the purposes of sign mounting height.

Support. Fig. 903.2.18.1 illustrates typical examples of the mounting height requirements contained for signs installed on U-Channel, Wood, PSST and Pipe Posts.

903.3.3.2 Mounting Height – Wide Flange (I-Beam) Posts (MUTCD Section 2A.18)

Support. Installing signs at the proper mounting height is critical not only for the sign to be seen and function, but also to the functionality of the breakaway design. Proper mounting height is more critical for breakaway function on Wide Flange posts compared to all other posts due to the hinge component of this post design. As with the other post types, mounting heights for Wide Flange posts are listed as “nominal” as excessive mounting heights have the same negative effects for these installations as exists with the other post types. Wide Flange post mounting heights are greater than other posts, so in areas with back slopes it is recommended to seek out a flatter location in advance or downstream of the original installation to keep the sign as low as possible.

Minimum mounting heights for Wide Flange post installations are not related to rural or urban classifications, but are directly related to how the breakaway system functions. Standard Plans 903 provides details on the nominal mounting heights on wide flange posts. Key details to focus on are:

• No wide flange post can be shorter than 7’ 9” measured from the hinge to the top of the stub.
• The hinge point is always below the lowest sign which is attached to the wide flange post.
• Nominal mounting heights vary depending if there is one sign mounted on the posts or two.
• For signs located in areas of back slopes, the minimum mounting height may have to be increased, or the sign installed in a different location, in order to achieve the minimum post length of 7ft. 9 in.

903.3.4 Ground-Mounted Sign Support Selection

Support. The majority of MoDOT signs are installed and supported on one of 5 types of ground-mounted sign supports or signposts. The selection of signpost is based on many factors, but primarily on the size of sign being installed and the type of roadway the sign is being installed along. There is some overlap in signpost applications; more than one signpost may be applicable to a given installation. The final selection of the post type is based on the attributes needed for a support as discussed in each classification of signpost below.

The number of posts needed to support a sign is primarily based on the width of a sign. Typically, signs 48 inches wide and wider are installed on two or more posts. This requirement is based on two factors, the capacity of the post and the long-term stability of the assembly. A wide sign installed on one post will place a torsional force onto a post and in windy conditions can result in an assembly not staying plumb and, in some cases, an actual failure of the post itself.

Standard. The selection of the proper size of signpost shall be based on the Signpost Selection Guide listed above. These tools will specify if a post type has the capability to support the sign in question and then specify what size post is required based on the requirements of the installation. Before the correct size of PSST or Wide Flange post can be selected, the length of the longest post must first be determined. To determine this, the offset and mounting height must first be determined.

903.3.4.1 U-Channel Posts

Support. MoDOT utilizes two primary sizes of U-Channel Posts, a 3 lb/ft high carbon, rerolled rail steel post for sign installations and a low carbon steel 1 lb/ft post for road side delineation.

U-channel posts can be used to support MoDOT’s small signs, such as no parking signs, object markers and chevrons on two lane roadways. U-channel posts are typically not suited to support larger permanent signs as they have limited torsional rigidity and have less ability to hold a larger sign steady in windy conditions. These are typically the most economical posts to use to support smaller signs and given these types of signs tend to be installed closer to the roadway their ability to yield more easily to impacts means they pose less of a damage risk to vehicles. U-channel posts are typically installed by driving the post into the ground without a stub or anchor, however, there is a stub / post installation option available which is detailed in the standard plans.

U-Channel posts are considered breakaway with no additional breakaway devices needing to be added. While there are breakaway devices available for U-channel posts, MoDOT’s use of this type of post for smaller signs typically doesn’t justify their use. A U-channel post’s breakaway is typically a yielding function, meaning as a vehicle impacts the assembly, the post yields and lies down in front of the vehicle so it can pass over the assembly.

Standard. U-channel posts shall be installed in accordance with the details found in Standard Plans 903. Signpost selection tools shall be used to determine sign sizes U-channel posts can support and the number of posts needed.

903.3.4.2 Wood Posts

Support. MoDOT’s specs permit three sizes of wood posts to be used: 4 in. x 4 in., 4 in. x 6 in., or 6 in. x 6 in. MoDOT’s wood posts are pressure treated to promote longer life and resist rot and insect damage. Wood posts were once MoDOT’s primary post to support signs on two lane roadways; however, due to issues with material stability PSST posts have become MoDOT’s standard post.

When used, wood posts are capable of supporting most sign assemblies on two lane roadways, from route marker assemblies, speed limit signs, warning signs and distance and destination signs. The use of a high quality wood post and proper installation is the key to a successful installation.

Guidance. The continued use should take into consideration the special characteristics listed in EPG 903.3 Ground-Mounted Sign Supports.

Proper installation is also critical for the stability of the sign assembly. The wood post should be placed a minimum of 36 inches into the ground, deeper for larger signs or in areas where the soil is weak or sandy, to keep the signpost plumb. When backfilling the hole, material should be added in lifts, or levels, in order to properly compact the backfill. Loose or fine materials, such as sand, sandy soil or dry concrete mix typically will not provide a long term solid backfill and can result in the post falling out of plumb over time.

MoDOT’s specifications should be followed when purchasing wood signposts. These specifications address a posts load capacity, breakaway attributes and the compatibility between the pressure treatment chemicals and our aluminum signs and sign hardware.

Option. While the soil originally removed from the hole can be used to back fill around the post other alternatives may be used, such as smaller quarry rock with the crushing fines mixed in, concreted mix or expanding polyurethane foam.

Support. Wood posts are considered breakaway without an add-on breakaway device; however, some sizes of post do need special preparation. 4 in. x 4 in. wood post are considered breakaway without any special modifications; however, 4 in. x 6 in. and 6 in. x 6 in. posts must be cross drilled at the base to weaken them so they will break away. The size of the holes and where they are drilled is critical to these posts meeting breakaway requirements. The details for these holes are found in the Standard Plans 903, it is important to note these breakaway holes are drilled in the sides of the post, not in the front of the post where the sign is mounted.

Standard. If wood posts are used, the proper size and number of posts shall be determined by using the post selection tools.

Fig. 903.3.4.2, Details for Wood Posts Requiring Breakaway Design

History. One of the earliest issues experienced with wood posts is their tendency to warp and twist, both before and after installation. Keeping a sign plumb and appropriately oriented to the roadway is critical to maintain the sign’s legibility and night time retroreflectivity performance. This aspect of wood posts resulted in significant waste of inventory when the posts warped and twisted before being used and increased work load on signing crews who had to correct warped and twisted posts after installations. Another concern with the use of wood posts was the installation required a hole to be dug, the posts set and property back filled so the sign would remain upright. If soil conditions prohibited a hole being dug deep enough or the back fill not capable for being compacted sufficiently the assembly would fall out of plumb. Along with these installation aspects, a wood post sign assembly can be very heavy, especially when the pressure treated wood is still wet with the pressure treating fluids and this can result in the need for additional people to set the post and/or increased risk of injury setting the post by hand.

Towards the end of MoDOT’s reliance on wood posts a new issue was identified relating to the more environmentally friendly treatment process called ACQ (Ammoniacal Copper Quaternary). ACQ replaced CCA (Chromated Copper Arsenate) for residential applications as CCA had chemical component which were not recommended for routine contact with skin. However, unlike CCA, ACQ (especially early versions) turned out to be very corrosive to metals, especially to aluminum. This corrosive nature requires special fasteners to resist this corrosive effect. Early applications of ACQ in other states realized serious sign corrosion to the point the sign would fall off the post in a matter of a few years. While it appears this has improved, special fasteners with special protective coatings are still recommended for use with ACQ posts. As a result, ACQ posts do not meet MoDOT’s specifications and should not be used to support signs. CCA treated posts are still MoDOT’s standard for wood posts, however, it is not commonly available at local home improvement centers and at many lumber yards. Due to MoDOT’s limited use of this product contract purchasing typically is not economical or possible.

903.3.4.3 Perforated Square Steel Tube Posts (PSST)

Support. MoDOT utilizes two sizes of PSST posts, 2 in. and 2.5 in., both being made from 12-gauge steel. PSST became MoDOT’s standard post for most sign installation applications on two lane roadways in the early 2000’s, replacing wood posts. PSST usage has since expanded to some applications on freeways and expressways.

Unlike U-channel or Wood posts, PSST utilizes a ground anchor, or footing, within which the post is then placed. MoDOT has several options in its specifications with respect to ground anchor/foundation systems, the use of each option is heavily based on the soil condition.

The anchor/footing types for PSST are:

• 12-Gauge PSST Anchor – this is the basic direct-driven anchor for 2 in. PSST posts. A 12-gauge anchor does not exist for 2.5 in. PSST posts.
• 7-Gauge Anchor – this is a heavy wall box tube anchor which is the basic direct-driven anchor for 2.5 in. PSST posts. It is also an optional heavy anchor for 2 in. PSST posts in rocky ground where a 12-gauge anchor may deform when driven. The 7-gauge anchor is also the anchor that must be used when installing a concrete or polyurethane footing.
• Omnidirectional, or stabilization, Anchor – this is the appropriate 12-gauge or 7-gauge direct-driven anchor with 4 soil stabilization plates added to the anchor to increase soil surface area to help keep signs plumb in weaker soils and/or in windy areas. A JSP will be needed to specify this anchor type on a project.
• Concrete Footings – In some applications it may be desired to install a concrete footing for PSST, similar to the footings for Pipe Post or Wide Flange Post. Concrete footings provide a stronger foundation compared to the directly-driven anchors listed above. A concrete footing may also be required in cases where the ground is too hard or rocky to direct drive the anchor and a hole may need to be dug in order to install the anchor. All concrete footing installations use a 7-gauge anchor (that only has holes at the top of the tube) because concrete would flow through the holes of a normal 12-gauge PSST anchor.
• Polyurethane Foam Footings – This is an alternate to a concrete footing for PSST post installations. This permits the footing and the sign to be installed in one trip compared to concrete, which requires a second trip to allow the concrete to cure. The installation requirements for an expanding foam footing are the same as a concrete footing except for the diameter of the footing, which is smaller. It is important to make sure the expanding foam used meets MoDOT specifications as not all foam products are acceptable to support a breakaway sign.

The connection between the PSST posts and anchors varies based on the anchor gauge:

• 12-gauge anchor – The connection between a 12-gauge anchor and the PSST post is accomplished using a corner bolt. The corner bolt pulls the post into a corner of the anchor and eliminates any slack or play between the post and the anchor.
• 7-gauge anchor – The fit between a 7-gauge anchor and the PSST post is much looser and the radii of each do not match so a corner bolt will not eliminate the slack or play between these two devices. Shoulder bolts installed at 90-degree angles corrects this issue; the shoulder of the bolt will pass through the holes in the 7-gauge anchor, but not through the holes in the post. As a result, the two bolts push and lock the post in two directions making a solid connection.
• Add-on breakaway devices – when required/used, the manufacture’s recommendations and hardware (if supplied) need to be used to connect the anchor, breakaway and post together.

Breakaway aspects of PSST are a little more complicated compared to other MoDOT posts, the requirement for an add-on breakaway device heavily depends on the size and number of posts needed to support the sign. It is important to follow the guidance found in the signpost selection tools and MoDOT’s standard plans to determine when an add-on breakaway device is required and when it is not. In applications where add-on breakaway devices are not required/used, PSST breaks away like a U-channel post in a yielding fashion, typically staying attached to the ground and lying down in front of the vehicle so the vehicle can pass over the assembly. However, when an add-on breakaway device is used the breakaway function changes and the assembly is designed to break away from the ground and permit the vehicle to pass under the airborne assembly.

Standard. If PSST posts are used, they shall be either 2 in. or 2.5 in. 12-gauge posts. The size and number of posts, as well as the requirement for add-on breakaway devices, shall be determined using the post selection tools. PSST posts shall be installed in accordance with Standard Plans 903. PSST posts installed on freeways, expressways, and ramps shall be installed using concrete footings.

Guidance. District operations should be consulted to determine the most appropriate footing for PSST posts alongside other roadways as footing requirements vary based on soil conditions.

903.3.4.4 Pipe Posts

History. In 2022, a pipe post capacity evaluation was conducted that resulted in a change to the pipe post load capacity and pipe post inventory. Historically it was believed that pipe posts could support a sign size of up to 30 sq. ft. but the evaluation determined pipe posts could support a sign of up to 58.5 sq. ft. The evaluation also determined that the 3 sizes of pipe post being utilized were redundant. MoDOT historically used 2 ½ in., 3 in., and 4 in. pipe posts, however, the evaluation determined that the sign capacity of a post is determined by the breakaway assembly. The 2 ½ in. and 3 in. pipe posts used the same breakaway design and therefore the 3 in. pipe posts did not have any additional capacity over the 2 ½ in. post. As a result, the 3 in. post is redundant and was discontinued. This decision allows for a simplified inventory and eliminates confusion on pipe size. Maintenance can continue to utilize 3 in. pipe posts until the inventory is depleted but shall not order new 3 in. pipe posts. All existing 3 in. pipe posts shall be treated as 2 ½ in. posts for purposes of choosing posts using the post selection tables. 2 ½ in. pipe posts can be installed on existing 3 in. stubs.

Support. MoDOT utilizes two sizes of pipe post, 2 ½ in. and 4 in. An important fact to understand is pipe post sizes are based on the inside diameter (I.D.) of the pipe post and not the outside diameter, this is the industry standard for pipe measurement. This is critical in selecting the correct pipe from inventory as well as charging out the correct post to keep your inventory levels correct.

Pipe posts have a similar sign capacity as PSST, even though they would appear to be able to carry a larger sign load due to size and thickness of the steel pipe. While the post themselves are far stronger than PSST, it is the breakaway of the pipe post which controls the sign load capacity of the post. The heavy-duty construction of a pipe post is not specifically related to sign load capacity but is more directly related to the durability of the post. Unlike PSST, which must be replaced after each vehicular impact, pipe posts are constructed with much thicker steel so the signpost can be impacted by a vehicle without being damaged and reinstalled for continued use. There are many pipe posts on our right of way that have been there for two or three generations of signs and are still functional so while they are heavier and more expensive, they are a long term investment and are far more durable.

Pipe posts are used for single and double signpost assemblies to support signs up to 58.5 sq.ft. These posts are typically used on freeways and expressways where signs are larger, wind speeds can be higher due to more open right of way and the sign may see larger snow load impact from plows pushing more snow from across multiple lanes to the right side of the roadway.

Pipe posts are also the preferred post to support large route assemblies, especially on freeways and expressways. In the past, Wide Flange posts were once used to support these assemblies (and many remain in place) as the design of the post was well suited to attaching a series of backing bars needed to support the assemblies. However, the multi-direction breakaway and high resistance to torsional or twisting forces makes pipe posts the preferred post over the Wide Flange design.

Pipe posts are designed and fabricated with the breakaway device as part of the post / stub combination; as long as the post and stub breakaway is assembled correctly the post is capable of being impacted from any direction. Details for the assembly of this post system are found in Standard Plans 903, special attention must be paid to the placement of three breakaway bolts, the required and proper placement of all washers within the breakaway and most critically to the proper tightening and torque of the breakaway bolts.

Standard. If Pipe posts are used, they shall be either 2 ½ in. or 4 in. in size. The size and number of posts shall be determined using the post selection tools. Pipe posts shall be installed in accordance with Standard Plans 903.

903.3.4.5 Wide Flange (I-Beam) Posts

Support. MoDOT uses 6 sizes of Wide Flange posts, commonly referred to as Design #1, #2, #3, #4, #5 and #6, increasing in size and capacity respectively. Wide Flange posts are typically used to support signs 30 ft2 and larger and are MoDOT’s highest capacity ground-mount sign support. As with Pipe Posts, Wide Flange post are designed to be a more durable post intended to last multiple generations of signs and designed to be able to be impacted by vehicle and then reassembled and reused.

Wide Flange post are designed and used to support large structural signs, signs made using extruded aluminum panels instead of flat sheet aluminum. The cross section of a wide flange post being that of an I-beam permits structure signs to be easily attached to the post using post clips or “dog clamps” instead of using traditional sign bolts. These posts are traditionally used on freeways and expressways only; however, there may be special applications where they may be used on two lane roadways if the size of the sign is too large for other post options.

Wide Flange posts were once the standard to support large route assemblies on freeways and expressways, however, over time two weaknesses were identified that changed this direction, making Pipe posts the better option. The two weaknesses of Wide Flange posts used to support route assemblies are:

• Safety - Route assemblies many times are installed in and around intersections and in these locations it is theoretically possible some assemblies could be impacted from any direction of travel. Single Wide Flange posts are only breakaway in two directions and are not designed to be breakaway if impacted on either side, just the front and back of the sign. Pipe posts are designed as a multi-breakaway post and can be impacted from any direction making them the better option for these installations.
• Torsional / Twisting Force Resistance – Although Wide Flange posts are very strong, they do have a limited resistance to twisting moments when installed as a single post installation. In wind prone locations, sign assemblies on a single Wide Flange post can begin to twist in the wind, and if this continues long enough, it can cause the post to fatigue and break off at the base. Pipe posts have an extreme resistance to twisting and can resist much larger torsional forces compared to wide flange posts.

As with Pipe Posts, Wide Flange posts are fabricated with the breakaway system as part of the post / stub assembly. While Wide Flange posts have a breakaway assembly at ground level like Pipe posts, they also require a hinge system located directly below the sign. The hinge system permits the wide flange post (the portion from the ground to the bottom of the sign) to swing up out of the way of a vehicle when impacted without the upper portion of the post and the sign needing to move. This reduces the mass that a vehicle must move when it impacts the post and in return reduces the impact energy to the car.

Unlike all other MoDOT posts, there are minimum post spacing which must be taken into consideration when selecting the correct number and size of post. Wide Flange Posts are much heavier than any other MoDOT post and hitting two of these posts at the same time in most cases would impart too much energy to the vehicle and would not meet minimum breakaway standards. These special considerations are included in Standard Plans 903 which contains all of the fabrication and installation details for Wide Flange Posts, however, due to their critical nature they are also listed here:

• Wide Flange Post Designs #1 and #2 have no minimum post spacing requirements.
• Wide Flange Post Designs #1 or #2 shall not be installed in three post configurations supporting signs less than 11 feet width.

• Wide Flange Post Designs #3, #4, #5 and #6 shall be spaced at least 7 ft. apart.

The post selection tool is designed to utilize two post installations over three post installations to help address minimum post spacing; it also reduces the number of footings which need to be constructed. However, there are some general rules based on sign size used to judge the number post for different size ranges of signs:

• Signs between 6 ft. and 17 ft. wide will typically be supported on two posts
• Signs wider than 17 ft. will typically be supported by three posts
• Signs of any size are not recommended to be installed on one Wide Flange post

Standard. If Wide Flange posts are used, they shall be either a structural #1, #2, #3, #4, #5 or #6 in design. The size and number of posts shall be determined using the post selection tools. Wide Flange posts shall be installed in accordance with Standard Plans 903.

903.3.5 Secondary Sign Supports – Post Extensions

Support. These supports are 3 in. aluminum I-Beam used to attach exit number panels to the top of, or to hang a secondary sign below, structural signs on new installations. Details of these posts are shown in the Standard Plans 903.

Option. There are occasions where modifications and/or additions must be made to existing sign installations where the existing posts are not long enough to support the new sign assembly. In these cases, it is permissible to utilize secondary sign supports to effectively extend the primary signposts to support signs a maximum of 3 feet taller than the existing primary signposts.

Secondary sign supports may only be used to allow taller signs to be installed on existing signposts if the signposts have the capacity to carry the larger sign based on signpost selection tools.

If a new sign assembly is more than 3 ft taller than the existing primary signposts, new signposts shall be installed.

903.3.6 Backing Bars

Support. Backing bars are typically used to support and stiffen wide flat sheet signs mounted on single signpost or to help support the individual signs which make up sign assemblies to form one unified sign assembly. Details for backing bars can be found in Standard Plans 903.

903.3.7 Breakaway Assemblies

Standard. All signposts installed on right of way shall meet federal breakaway standards and MoDOT standards. Signposts not meeting current standards, but met the standards at the time of their installation, may remain in place until the end of their service life.

Sign trusses and other large sign support structures that are not breakaway shall be protected by acceptable shielding, such as guard rail or barrier wall.

Support. 4 in. x 4 in. wood posts do not need any modification to be breakaway, however 4 in. x 6 in. and 6 in. x 6 in. wood posts will need to be cross drilled to meet breakaway standards. U-Channel posts do not require breakaway modifications if they are direct driven into the ground, however, if the ground stub and slice installation method is used the installation will need to be installed according to the Standard Plans 903 to meet breakaway requirements. PSST will require breakaway devices added in certain applications based on sign and number of posts used for an installation. The signpost selection tools will indicate when a breakaway is required for PSST posts. Pipe and Wide Flange posts have the breakaway devices integrated into the post design.

903.3.8 Sign Orientation (MUTCD Section 2A.20)

Support. The orientation of the face of a sign in relation to the driver and roadway is critical to visibility and legibility, especially at night. The effectiveness of the retroreflective sheeting on a sign can be negatively impacted if the orientation of the sign face is not correct, due to incorrect installation and/or a signpost being damaged and knocked out of alignment.

The orientation of a sign can also help reduce unwanted reflection or glare off of the sign face. The skew angle, shown in Standard Plans 903, is designed to help address this glare issue for tangent sections.

Guidance. Unless otherwise provided in the EPG, signs should be vertically mounted at approximately right angles to the direction of, and facing, the traffic that they are intended to serve, with an approximate 3 degree skew angle away from traffic.

Signs that are placed 30 ft. or more from the pavement edge should have a skew angle that is turned toward the road to improve the effectiveness of the retroreflective sheeting.

Option. While the standard skew angle is 93 degrees, the skew angle may be adjusted to maintain brightness and avoid glare for signs on curved sections of road. On roadways with significant grades, sign faces may be tilted forward or back from vertical position to improve the viewing angle.

903.3.9 Sign Mountings (MUTCD Section 2A.21)

Support. Attaching a sign properly to a sign support is critical in order to properly orient the sign in relation to the driver as well as provide a durable, long life installation.

Standard. Plastic/nylon washers shall be used between the heads of all twist fasteners (such as screws, bolts or nuts) and the sign face to protect the sheeting from the twisting action of the fasteners.

Signs shall be attached to each type of sign support in accordance with Standard Plans 903.