903.4 Overhead Sign Mounting
Guidance. (MUTCD Section 2A.17) Overhead guide signs are warranted at many important locations and should be used on freeways and expressways, at locations where some degree of lane-use control is desirable and where space is not available at the roadside. It is recommended that justification be provided when mounting signs overhead.
Support. The factors to be considered for the installation of overhead sign displays are not definable in specific numerical terms. Refer to Sign Supports for further information.
Option. Warrants are not specific, but the following conditions, listed in no particular priority, should be considered:
- Traffic volume at or near capacity
- Complex interchange design
- Three or more lanes in each direction
- Restricted sight distance
- Reduction of basic lanes
- EXIT ONLY lane drops
- Left exit ramps
- Closely spaced interchanges
- Multi-lane exits
- Large percentage of trucks
- Background of street lighting and advertising signs
- High-speed traffic
- Insufficient space for ground mounted signs
- Consistency of sign message location through a series of interchanges
- Junction of two freeways
If a mix of situations exists on a route, where some locations require overheads and others do not, then all guide signs on the project are mounted overhead. Mixing ground-mounted guide signs with overhead guide signs is discouraged. Supplemental guide signs for traffic generators are not mounted overhead for any highway system and are not considered when determining whether to mount all guide signs overhead.
Over-crossing structures may serve for the support of overhead signs and, under some circumstances, may be the only practical solution that will provide adequate viewing distance. Use of such structures as sign supports may eliminate the need for the foundations and sign supports along the roadside.
903.3.1 Locating Support Structures
Foundations of any type are not placed in pavement gores or other exposed locations. Every attempt is made to avoid special structure designs. Whenever possible, sign support structures are not mounted on bridges or walls, and are placed in areas which may already require guardrail or traffic barrier due to other obstacles.
Box span trusses (refer to Standard Plans 903.10 or 903.60) may also support a cantilever support beyond one or both columns. This may help eliminate the need for a structure for the opposing traffic direction. Guide signs may be placed over the left hand lanes to utilize a span-cantilever design.
Structure locations should be checked against existing and proposed underground drainage and utilities for potential conflicts. Structures in paved medians with concrete traffic barrier shall be located at a minimum of 150 ft. from median inlets to allow for barrier transitions and field adjustments of the underground storm culvert, if necessary.
To minimize glare and sign obstructions, structures are located so signs do not face roadway lighting units and traffic signals. When limited geometrics require numerous traffic control devices within a given area, a 300 ft. minimum is maintained between the sign face and the stop bar location. This includes structures located before or after the stop bar. For lighting units, a minimum recommended distance between the front of a sign and the luminaire is 75 ft. Signals, lighting and sign structures should be coordinated to achieve the best layout and remain within the standards indicated in Highway Lighting and Traffic Signals.
Roadway geometrics may cause sign glare at night from vehicle headlights on parallel outer roads, service roads or the mainline. Headlights from a mainline vertical crest curve located before overhead signs may create sign glare, and, if possible, structures should be located to reduce or eliminate this glare.
Roadway curvature is considered when placing overhead guide signs with down arrows (Type C). Down arrows on the outside of a curve may give the illusion of pointing to a lane other than intended. Down arrows on the inside of a curve may appear to point down at the shoulder or side slope. Consideration should be given to move overhead guide signs out of curves, or provide additional advance overhead signs on tangent sections of roadway.
903.3.2 Horizontal Clearances
The preferred location of the exposed foundation is outside the clear zone. Normally, this may only be accomplished with span structures. When this clearance is not economically feasible, the minimum horizontal clearance to the nearest part of the exposed foundation is designed with an appropriate barrier according to the standard plans and the AASHTO Roadside Design Guide. The minimum lateral offset from the edge of the shoulder (or if no shoulder exists, from the edge of the pavement) to the near edge of overhead sign supports (cantilever or sign bridges) shall be 6 ft. Cantilever structures with appropriate guardrail design are used to mount single guide signs at the exit gore location.
903.3.3 Vertical Clearances (MUTCD Section 2A.18)
Support. The provisions of this article apply unless specifically stated otherwise for a particular sign elsewhere in this manual.
A minimum vertical clearance of 17 ft. 6 in. is maintained between the highest point of pavement or shoulder (including mountable curbs located within the shoulder limits) and the lowest point of the sign or truss. This applies to all types of overhead structures. Bridge-mounted signs are no lower than the bridge, or 17 ft. 6 in., whichever clearance is lower.
Signs may be raised above the standard vertical clearance to improve visibility and avoid other obstructions such as signal equipment.
Standard. Vertical clearances for single wide flange and all post designs are set from the driving edge of the travelway to the bottom of the lowest sign. Vertical clearances for multiple wide flange posts are set from the driving edge of the travelway to the bottom of the lowest sign, or the length of the shortest post, whichever requirement provides the higher clearance. Vertical clearances for different designs are shown in the standard plans.
Supports for sheet signs, such as regulatory, warning and route shield markers are designed to provide a nominal vertical clearance of 7 ft. with a minimum post length of 6 ft. This will allow for field adjustments to maintain a required clearance of 6 ft.
All two- and three-post guide signs (any LOGO sign or sign with white legend on green, blue or brown sheeting) have a minimum 7 ft. 6 in. clearance from the driving edge of travelway. Also, the shortest post is a minimum of 7 ft. 9 in. from the ground line to the bottom of the lowest sign. This will provide sufficient post length to allow hinge plate installation, provide a 7 ft. clearance for errant vehicle impacts, and a 6 in. field adjustment. Secondary signs placed directly under a guide sign have a minimum design vertical clearance of 5 ft. 6 in. from the nearest edge of travelway, the shortest post is 7 ft. 9 in. from the ground line to the bottom of the lowest sign, and the main guide sign is a minimum of 8 ft. 6 in. from the nearest driving edge of travelway. Meeting one of these three requirements will exceed the other two minimum clearances.
Due to wide flange post limitations, large vertical clearances of 25 ft. or more are avoided by either moving the sign or considering a truss-mounted sign.
All two- and three-wide flange post signs contain hinge plates as shown in the standard plans. On the shortest post a vertical clearance of 7 ft. 9 in. is maintained from ground line to the bottom of the lowest sign. This will allow sufficient post for a hinge point cut line 3 in. from the bottom of the lowest sign. Maintaining this clearance on the shortest post is critical for exit gore and other signs, which are located between roadways and have high exposure to traffic.
Option. The height to the bottom of a secondary sign mounted below another sign may be 1 ft. less than the height specified above.
Where signs are placed 30 ft. or more from the edge of the travelway, the height to the bottom of such signs may be 5 ft. above the level of the pavement edge with a 5 ft. minimum post length.
A route sign assembly consisting of a route sign and auxiliary signs may be treated as a single sign for the purposes of this article.
All posts except structural posts should have a minimum clearance of 5 ft. above the driving surface with a minimum post length of 5 ft. above the ground. The mounting height may be adjusted when supports are located near the edge of the right of way on a steep backslope.
Support. Without this flexibility regarding steep backslopes, a decision to relocate the sign closer to the road could be made, which might be less desirable.
Standard. Overhead mounted signs shall provide a vertical clearance of not less than 17 ft. 6 in. to the sign, light fixture, or sign bridge, over the entire width of the pavement and shoulders except where a lesser vertical clearance is used for the design of other structures.
Option. If the vertical clearance of other structures is less than 16 ft., the vertical clearance to overhead sign structures or supports may be as low as 1 ft. higher than the vertical clearance of the other structures.
In special cases it may be necessary to reduce the clearance to overhead signs because of substandard dimensions in tunnels and other major structures such as double-deck bridges.
Support. Figure 903.1.23.1 illustrates some examples of the mounting height requirements contained in this portion of the article.
903.3.4 Guardrail
Type A guardrail designed according to the standard plans and the AASHTO Roadside Design Guide is used to shield traffic from structure foundations. Type E guardrail is used only when necessary for areas with limited horizontal clearance, and installed within the guidelines as shown in the standard plans. Structures are designed to eliminate the need for Type E guardrail.
Consideration may be given to placing guardrail at the edge of the clear zone to protect traffic from pedestals placed outside the clear zone.
903.3.5 Span, Cantilever and Butterfly Box Trusses
Aluminum and steel span, combination span-cantilever, butterfly and cantilever box trusses are shown on Standard Plans 903.10, 903.12 and 903.60. These types are used when the required span exceeds the 90 ft. maximum span of standard tubular supports, or when sign height, sign design area, or sign spread exceeds the limits of tubular support designs. Necessary information to complete truss data sheets for overhead trusses include:
Design data for both aluminum and steel span box trusses are shown on Forms D-32 and D-33. The contractor has the option to use steel or aluminum for span designs. Steel box trusses are used without option for all types of bridge-mounted trusses, and for all butterfly and cantilever trusses.
The standard foundation design in Standard Plan 903.12 is a drilled shaft. A spread footing foundation design may also be used. Savings may be realized by basing the foundation design on actual sign area rather than maximum and also using site specific geotechnical information. Upon request, Bridge will determine the most cost effective design. Note that ample time must be allowed for this determination. See Guidelines for the Design of Miscellaneous Foundations in Bridge Reports and Layouts for the proper procedure.
Truss designs for spans exceeding 160 ft., bridge, wall and any special mounting conditions are requested from Bridge. See other areas in this section for truss design procedures involving the Bridge Division.
Column height, "H", for box trusses is shown to the nearest inch (0.05 m) on the truss cross-section sheet. Exposed foundation heights are designed as shown in the standard plans. This height may be adjusted up 6 in. or down 3 in. during construction to meet field conditions. This will avoid any change in design height "H" of the prefabricated steel columns. An assumed elevation of 100 ft. at the highest point of the roadway is used to create the truss cross-section, and the scale is 1 in. = 5 ft. (1:50). An assumed elevation will allow the movement of a truss over an area with the same template without adjusting all the elevations. Elevations to be shown (referenced to the assumed elevation) on the cross-section include: top of foundation, top of footing (as applicable), bottom of base plate, the centerline elevations of truss and signs, the top of the top chord and the bottom of the bottom chord. The elevation difference between the bottom of base plate and top of pedestal for placement of expansive grout is approximately equal to the anchor bolt diameter + 0.5 in. Refer to the standard plans for anchor bolt diameters. The truss cross-section contains, at a minimum, the following information: truss number, station, roadway, column types, sign design area, overall truss length, foundation information, sign locations, guardrail, and any signs to be strapped to the truss column. Columns are numbered from left to right. Different column types are allowed for each column. Truss sections are drawn as the driver views the main sign(s). If applicable, signs are indicated as near and far side, the far side sign being shown with dashed lines. Lighting is not provided for any overhead sign. Provisions are made for hand holes and foundation conduit, as shown in the standard plans, in case lighting is needed in the future.
Examples of cross-sections for cantilever, butterflyand span box trusses are available. Combination span-cantilever truss cross-sections require similar information.
903.3.6 Tubular Steel Sign Supports
Tubular steel span, cantilever and butterfly sign supports are shown on Standard Plans 903.05, 903.06, 903.07 and 903.08. Details necessary to complete Form D-34 (see Figure 903.18.1.5.3, 8-03.8) are shown on the standard plans. These structures have limiting design factors including sign height, total sign area, and sign spread, as noted on each drawing. When any one of these limits is exceeded, box type trusses are used.
Required information on cross-sections is similar to box truss cross-sections. For two arm spans between 40 and 75 ft., post lengths are in 3 in. increments. Post lengths for other designs are in 1 ft. increments. The foundation height above the ultimate ground line is designed as shown in the standard plans. During construction the foundation height may be adjusted up 6 in. or down 3 in. to match actual field conditions. This will avoid changes in the prefabricated post lengths and accommodate actual field conditions. The quantities of concrete for tubular steel support footings have been tabulated and are also shown in the standard plans.
An example of a one-arm span tubular structure cross-section is available. Other types of tubular steel structure designs require similar information as shown on this figure.
903.3.7 Bridge- and Wall-Mounted Signs
Bridges are utilized wherever possible to mount guide signs and, possibly, sign trusses. This concept removes a potential roadside obstacle and any need for guardrail or concrete traffic barrier, and is generally more economical. The district requests Bridge to prepare details for bridge-mount sign support brackets after preliminary sign location approval. Bridge prepares plans that are included in the appropriate bridge details. Bridge is responsible for bid items and design of bridge and wall-mounted sign support brackets on new bridges. When retrofitting an existing bridge with a new bracket, Bridge designs the bracket and submits the plan to the district for inclusion into the roadway plans.
The sign and sign brackets are designed to provide a clear distance of 2 ft. from the outside edge of the bridge to the edge of the sign. The sign has a maximum skew angle of 5 degrees to the roadway for which it is provided. The sign bracket is designed so the sign is visible to traffic at least 600 ft. ahead of the installation.
903.3.8 Briges- and Wall-Mounted Structure Designs
The Bridge Division designs all bridge- and wall-mounted structure pedestals.
For wall- and bridge-mounts, the district prepares and submits a sign support structure cross-section to Bridge for their concurrence of location, column type and height. The sign dimensions and location over the roadway are included with the request. The district coordinates design details with Bridge so pedestal details are included in the bridge or wall plans. Bridge is responsible for design of pedestals for bridge- and wall-mounted structures and related pay items. The district is responsible for the design of the structure above the pedestal and related bid items.
When retrofitting an existing bridge or wall with a new sign support structure, the preferred location and existing bridge or wall number is submitted to Bridge, along with the same data as required for a new bridge-mounted sign support structure. The incorporation of bid items is similar to new bridge-mounted sign support structures.
903.3.9 Non-Standard Sign Support Structures
Special design details are required on spans greater than 160 ft. and other situations not covered by the Standard Plans. A truss cross-section is submitted to Bridge as required for any new truss. The district receives recommendations from Bridge for special foundation, truss, and column information. Design of non-standard structures should be in accordance with the 2001 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals and latest interims. Unless otherwise noted, the district is responsible for including the information in the final plans.
903.3.10 Sign Supports on Fill
The pole’s design height for overhead sign supports placed on elevated roadway fill is to include the height of the fill. Consult Bridge if the pole’s design height exceeds the values shown in the standard plans.