The purpose of this draft worksheet of specifications is to provide assistance and guidance to those preparing to provide warning, over and above those warnings required by state and federal law, to drivers of overheight vehicles or cargo loads.

The value of providing special warning to drivers of overheight vehicles is the prevention of accidents caused by driver error or inattention. 

The most effective means found to date for providing special warning, is to individually alert the driver concerned and make them specifically aware that they will hit the low overhead object if they proceed.

In accidents caused by overheight vehicles, it is often the unoffending drivers and passengers of other vehicles, secondarily involved, that suffer the greatest injury. The fact that the driver of the overheight vehicle may be responsible and may pay for damages is of little importance in this event. The costs associated with the resolution of one accident even when involving a single vehicle generally exponentially exceed the cost of installing an Overheight Vehicle Detection and Warning System.

Answers to specific questions or assistance in planning solutions to exceptional problems is available on request from Trigg Industries.

Preparation of Specifications

Suggestions to specification writer: 

  1. Read through these specifications, crossing out paragraphs and words that have noapplication to your problem.
  2. Modify or add to the remaining text as your situation dictates.
  3. Reread the final product to assure that all points are adequately covered in the mannerprescribed by local law or practice.



The design described herein is to prevent overheight vehicles/loads from striking the (bridge),(tunnel entrances), (etc.) by vehicles traveling (north-south bound), (east-west bound) on highway ____________________ in the direction(s) of ___________________ at point(s) ___________________ .

Detection and warning items are to comply with (State) regulations and the (U.S. D.O.T. F.H.D. Manual of Uniform Traffic Control) devices. All items shall be located to provide warning early enough (in consideration of “practiced vehicle speed” at the site) to allow sufficient time for driver to react. There should be maximum safety for all drivers with minimum disruption of traffic flows.


All equipment shall be supplied to ___________________ for installation by ___________________. Each component of the equipment shall be accompanied by full instructions for installation, wiring, assurance of proper functional interface of components and other information needed for installation and functional testing. 


A. (State) legal requirements shall be met by inspection of or installation of standard signs as required by ___________________ (list if necessary) ___________________. The necessary actions for this purpose shall be accomplished by ___________________.

B.Warning of drivers of vehicles specifically detected as being overheight for clearance of the overhead structure shall be as follows: 

  1. DETECTION - All overheight vehicles or vehicle loads traveling toward the above described structure shall be detected and specific individualized warning shall be given Vehicles traveling away form the structure shall be ignored by the detection equipment. Detection of vehicles traveling toward the structure shall be counted and recorded on an electronic counting device. The detection system shall consist of visible red or infrared source(s) and spectrally matching detector(s) mounted on poles positioned on opposite sides of the approach roadway at a distance of __ feet ( __ meters) from the structure. The alignment and height of the visible red or infrared beam shall be preset to identify vehicles or loads of over __ feet ( __ meters), __ inches ( __ cm) above the roadway at the point of detection.

NOTE FOR SPECIFICATION WRITER - The specified height should be the lowest clearance between the road surface and the overhead structure, less either the clearance required by state law or the allowance required for clearance after vehicle bounce etc. has been considered plus the amount deemed necessary for settling or margin of error.

    2. ATTRACTION OF DRIVER ATTENTION - Positive action shall be taken to attract the attention of the driver to the fact that the driver specifically will have an accident if the driver proceeds. The action must cause him to become alert to the extent that he reacts as instructed. Caution should be taken to cause the least possible disturbance to other motorists and to minimize the adverse effects of increased ambient sound or light in the area.

    3. WARNING AND INSTRUCTIONS FOR DRIVERS - Warning and instructions to drivers of overheight vehicles shall be stated on the face of the warning sign and shall be illuminated internally, i.e., nature of danger “OVERHEIGHT” and action to be taken (“STOP” or “TURN RIGHT” or “EXIT RIGHT” or “USE RIGHT (LEFT) LANE,” or “DO NOT ENTER” etc.) if the driver proceeds. The action must cause the driver to become alert to the extent that they react as instructed.

NOTE TO SPECIFICATION WRITER - If the instruction line on the sign is to read “STOP”, there may be a need for an additional sign to instruct the driver on actions to take after they have stopped. This sign, if needed, should be located where the driver will see it unavoidably, after stopping. It should be in letters large enough for the driver to read but small enough not to attract the attention of drivers who are not involved. The theory behind the requirement for this sign is that the driver, who has been stopped, is possibly in a state of confusion and may act in such a way as to cause a new danger to other motorists.



  1. Equipment is hereinafter referred to by manufacturer for the purpose of establishing standards of quality for Overheight Vehicle Detection and Warning Systems manufactured and/or supplied by Trigg Industries, are presently considered as acceptable when conditions of the drawings and specifications are met.

    Products of other firms offered will be considered, subject to the Engineer’s approval and will be based on the quality and capability of the substitutes compared to the equipment available from the above indicated supplier.

  2. All equipment shall be constructed in a workmanlike manner and present a neat and finished appearance when completed. Proper operation of the equipment shall commence immediately after restoration of power.
  3. Safety Features - NEMA 3R rated metallic equipment enclosures shall be provided with terminals for attachment of ground safety wire.



The equipment shall operate and meet all of the requirements of this specification under the following atmospheric conditions:

Temperature: -40ºF to 135ºF (-40ºC to 57ºC).

Relative Humidity: 0 to 100%.

Rain: 2 inches per hour at 200 feet between source and detector.

Snow: Light snow.

Fog: 200 foot visibility.

Wind Velocity: 0 to 90 miles per hour (0 to 144 km per hour).


The equipment shall operate properly when the sun is outside 10 degrees of the extended optical axes of the receiver unit in its installed configuration. If the above requirements cannot be met, the equipment will be deemed satisfactory if explicit installation information is given such that the rays of the sun cannot interfere with the proper operation of the equipment. This provision includes reflections from vehicles.


The variation in light intensity caused by the shadow of passing clouds shall not interfere with the proper operation of the equipment.



The detector unit electronics shall be solid state with printed circuit boards and regulated power supply. The unit shall be modular assembly type. It shall have an effective range of 10 feet (3 m) to 200 feet (38 m) with a reaction speed range of 1 MPH (1.61 km/h) to 75 MPH (121 km/h) for a 2½ inch (6.35 cm) diameter object 1 inch (3 cm) above the detection height.

It shall contain provisions for the elimination of the effect of ambient light and an internal environmental control element that reduces operational failure from fog, condensation and insects. Dimensions shall not exceed a maximum overall size of 19 x 15 x 10 inch (48 x 38 x 25 cm). The housing shall be of cast Almag of not less than 1/8" thickness and shall be weather sealed. The mounting shall allow for directional adjustment and aiming after initial installation.


Two 1/8 inch (.32 cm) boresight holes are located at top-middle of each housing. Front and rear screws are installed in these holes so as to insure a weather tight enclosure, and should be removed to allow alignment of units. Looking through the boresight hole from rear of unit so as to see through the boresight hole in front of unit gives the installer a basic means of aiming the unit in the proper direction. Focusing the unit on opposite sides of the road in the center of the front boresight while looking through the rear boresight hole ensures that a general alignment is accomplished. This step should be done from both transmitter and receiver unit locations. Fine tuning of alignment can then be done electronically.


The transmitter unit and the receiver unit shall each be provided with a barrier to protect the operating equipment. The enclosure shall maintain its structural integrity for the operational life of the equipment and shall allow ready access for control adjustment and electrical interconnection without the use of any tools except a phillips head screwdriver.


Each of the equipment units shall be provided with means for rigidly attaching the unit to a vertical cylindrical pole without requiring any machining operation. The attachment means shall not stress or deform the unit and shall prevent the movement of the unit in any direction by the force developed by wind. The mounting means shall allow adjustment of the vertical position on the pole. The mounting means for the transmitter unit and the receiver unit shall have the capability of adjusting the angular orientation of the optical axis in both the horizontal and vertical plane over an angular range of plus or minus five degrees from horizontal. The transmitter and receiver unit shall be mounted to detect the presence of vehicles that exceed the specified vertical height.


Signs shall meet all electrical/electronic specifications stated in sub-paragraph IV.A.2 above and shall conform, in all essential elements, with the provisions of the U.S. D.O.T., F.H.A. Manual on Uniform Traffic Control Devices. Plastics on sign faces shall be protected by LEXAN™ or other material providing equal or greater defense against flying object damage or vandalism. Signs shall be activated and time controlled by the detection unit. Lighting shall be provided by high output fluorescent bulbs for message and incandescent bulbs with parabolic reflectors for arrows at the top and bottom of sign.Housing shall be within 10% of horizontal and vertical dimensions as defined (either rectangular or diamond shape) with reinforced aluminum capable of withstanding wind loading up to 90 MPH (144 km/h) and provided with a sun shield protecting the display Letters and arrows shall be normally blanked out. When illuminated, arrows shall be amber in color, but can be made in any color specified.


Signs shall be modular amber LED arrays with a viewing angle of 30 degrees. The pixel luminance shall be such that the sign is viewable in direct sunlight. The LED modules shall be factory programmable with a custom message and character font within the parameters of the 8 inch by 14 inch pixel array. Pitch shall be at least 1 inch between pixels. Each pixel shall have at least 4 LEDs such that the failure of one LED will not cause total loss of the pixel. The sign enclosure shall be NEMA type 3R, fully front- serviceable aluminum. The sign shall be activated by the detection unit and will include an alternating flasher and relay for activating associated LED traffic heads or audible alarms. Than standard sign input power shall be 120VAC. The sign shall optionally provide input power of 240VAC or 12VDC for solar applications. Proper sign operation shall be sustained across a -40 to +22°F temperature range. The sign shall be mounted using a double pole design and have a lifting means that facilitates installation.


VMS devices shall be provided with either Amber, Red or Red/Green/Blue (full color display) full matrix LED’s depending upon the application. Character height “rule of thumb” is generally 1" for every 50’ of viewing distance required. The number of lines and message desired will determine the physical size of the VMS. Viewing angle is generally 30 degrees but a 70-degree viewing angle may sometimes be required. Photocell control for dimming based on ambient light shall be provided. The “Default” message may be blank or updated either locally at the VMS using a laptop computer or remotely via dial up/wireless modem, fiber optic link or network communication using a desktop computer. The controller between the VMS and OVDS or computer shall be NTCIP compliant. The overheight warning message shall have priority over the default message and shall be displayed for the duration of the Alarm Time setting in the OVDS detector. Software for VMS message control shall be Windows compatible. The VMS devices shall operate with a power of 120 VAC +/- 10% and a temperature range of –40 to +138 degrees F with internal fans for cooling. The VMS case shall have a NEMA 3R rating and be fully front-servicable. VMS devices may be pole(s) or wall mounted but the user is responsible for all on-site mounting.


Shall be activated and time controlled by the Detection Unit. The Parabolic Bell is 38 inch (97 cm) in diameter and constructed of fiberglass. Electrical input shall be 120 VAC +/- 10%, at50/60Hz. Parabolic Shields or other sound controlling directional devices shall be capable of withstanding wind loadings up to 9O MPH (144 km/h). Mounts shall be adjustable in vertical plane and adaptable for attachment as required herein.


Shaft - The shaft shall be a two piece, adjustable height, seamless, round aluminum tube. Poles shall include a handhole centered 18 inch (46 cm) above the bottom of the shaft. A cover, with stainless steel attachment screws, shall be provided for the handhole.

Base Flange - The base flange for the attachment of the shaft to the foundation shall be a one piece cast socket of aluminum alloy. Concrete footing and four anchor bolts shall be provided by .

Pole Fittings - Must be specifically designed to accomodate the overheight detector, bells, signs, etc. specified herein whether supplied by the manufacturer of electronic components or of the poles. Poles come in two styles: 10 feet (3 m) non adjusting or 10 - 16½ feet (3 - 5 m) adjusting (there are two pieces or shafts with this pole).

NOTE TO SPECIFICATIONS WRITER - Any sturdy pole or vertical structure can be used, including those already in position for other purposes. The above paragraph must be re-written to fit your plan keeping in mind the necessity for specifying how the equipment will be attached.. 


Power and signal cabling shall be provided between the receiver unit electronics and the junction box adjacent to the mounting pole foundation. Power for the receiver and transmitter units will be available at the junction box located adjacent to the mounting pole foundation and shall be made available by(WHOM).


See attached “OVERHEIGHT WARNING SYSTEM CABLE WIRING DIAGRAM” and the above quality and performance specifications.