Bridge strike

Some countries have standards on minimum vertical clearance of roadway. Any structures that do not meet that clearance would need to have warning signs. United Kingdoms has a standard on minimum clearance of a public highway at 16 feet 6 inches (5.03 m). Any bridges that do not meet the clearance requirement are considered to be "low bridges" and they require to have signage to indicate the clearance.[18]

In United States, the Manual on Uniform Traffic Control Devices indicates that any structures that have vertical clearance of less than the legal maximum vehicle height plus 12 inches (30 cm) require to have low clearance warning signs. When warning signs are installed on the structure, it can be either in diamond or rectangle shapes. For advanced warning signs, it must be in diamond shape.[19] There is no federal standard for legal maximum vehicle height. This is set by the states. The most common vehicle height limit is 13 feet 6 inches (4.11 m) used in 32 states on the eastern side. This effectively sets a requirement that structures in those states that have clearance of less than 14 feet 6 inches (4.42 m) must have low clearance signs. Higher legal maximum vehicle height limits used in other states.[20][21]

There is a variety of shapes, colors and designs used for low clearance and height limit warning signs, indication signs, and prohibition signs used wordwide.

In United Kingdom, there is an emphasis on making low clearance structures be more noticeable to drivers. This includes having large "LOW BRIDGE" letters and hazard marking on the low bridges. It has various warning signs including warning signs at or near the structures, advanced warning signs, and road markings. For advanced warning signs, there can be large panel with alternative route information.[18] Similarly in Australia, advanced traffic instruction signs are used to provide clearance information of a low-clearance bridge ahead and accompanied by a detour direction.[22][23]

• Warning signs
• 
  Low clearance warning sign that is hard to see
• 
  Large "low bridge" sign with hazard marking on the bridge
• 
  Low clearance advanced warning sign
• 
  Advanced warning with alternative route information

Speed limits along with road narrowing, speed bumps, rumble strip or other traffic calming techniques can be used to force reducing vehicle speeds to prevent bridge strikes.[2]

Tattle tales are series of chains suspended from an overhead gantry over the roadway. The gantry is also equipped with warning signs and other markings. The chain are set to the clearances of the approaching low-clearance structures. When any parts of vehicles hit the chains, it creates noises to notify drivers so they can pay attention to the height restriction signs.[24][25] Sometime a bar is hung at the bottom of the chains to make it more noticeable to drivers.[26] This type of devices only works in low speeds. High speed vehicles can run through without giving out enough noticeable noises to the drivers.[2]

In special situations where the clearance of a structure is reduced longitudinally, the beginning part of the structure may give an appearance that it provides enough clearance for vehicles to go through. As the vehicles are driven under it, the clearance is reduced. The vehicles may be wedged and stuck under the structure as it is too late for drivers to realize that it is not enough clearance to go all the way through. In this case, false soffits can be added to the beginning part of the structure. The false soffits are flaps that extend down from the underside of the structure to match the lowest clearance of the entire structure to provide visual aid to drivers of the actual clearance that they will be facing. There can be hazard markings on the false soffits to make them highly visible.[18]

Other devices include scarifying structures. These structures are installed in front of the bridges or tunnels with the heights of the clearance to absorb the impacts of overheight vehicles. The primary goal of using scarifying structures is to prevent damages to the bridge and tunnel structures.[2] Height restriction bars can be installed ahead of the structure to ensure that overheight vehicles will not be able to approach the low-clearance structure.[27] Collision protection beams are installed right in front or at the structures. They take the impacts and dissipate the energy from bridge strikes to prevent damages to the structures.[18][28] As the goal of these devices are to protect the structures, they are used as the last resort without providing any benefits to crash prevention. Without other devices, risks to vehicles still hold.[2] In 2019, a bus crashed onto a height restriction bar in Dubai, resulting in several fatalities.[29]

• Passive devices
• 
  Tattle tales to warn of an approaching low bridge
• 
  False soffit to match the lower clearance ahead
• 
  Height restriction bar in front of this covered bridge
• 
  Collision prevention beam installed in front of this bridge

Warning sign with yellow flashing lights can be triggered by an overheight vehicle detection

Overheight detectors integrated with flashing lights and other warning signs are used to in some locations to provide an active warning to the drivers of the vehicles that are over the height restrictions of the upcoming bridges or tunnels.[22] Despite the yellow flashing lights, this method may still fail to gain attention of the drivers. The infamous Norfolk Southern–Gregson Street Overpass (also known as the 11-foot-8 Bridge) had 100 crashes on record between 2008 and 2016 even with this type of systems.[30][31] An improvement to this is to have the detectors fully integrated with traffic lights to stop all traffic when an overheight vehicle is detected.[30]

Police officer measures truck's height after the red traffic light is triggered to stop all traffic

Fully integrated systems of overheight detectors and traffic lights have been used in tunnel operations for decades. By the 1970s, several tunnels in the United States had such systems in place.[26] An example was a systems at the Hampton Roads Bridge–Tunnel which had two detection points. The first point provided an audible and visual warnings to drivers to pull over to an inspection station. The vehicle would be measured by an authority before allowing to proceed. In an event that an overheight vehicle failed to stop at the inspection station, the secondary detector would trigger another alarm that would display a message at variable-message signs to lower the speed limit and eventually turned the red light to stop all traffic.[32] Most detectors in the 70s used photocell system with light source installed on one pole and a photocell detector installed on opposite side of the roadway. A major drawback of the photocell detectors was that they produced a lot of false alarms caused by snow and heavy rain. Another type of detectors at the time included trip wires that would be broken if an object at that height hit the wire.[26]

Modern detectors now use other technologies such as red/infrared photoelectric sensors, infrared laser beams, and ultrasonic transducer. Some are dual-beam systems to increase reliability by allowing the system to function even with single beam, and to lower false alarms by rejecting birds and other materials. More sophisticated systems include camera systems that can capture 3D profiles of vehicles in realtime, and microwave radar systems that can capture vehicle height profiles.[33][2] Contact-based detection systems are also in use such as low-clearance detection bars that are movable vertically to initiate a trigger.[33]

In case of overheight vehicle detection systems that are fully integrated with traffic light control systems but unmanned, there is no personnel available to perform measurements and direct overheight vehicles away from the bridges. The lights will turn green after a short stop with an expectation that the drivers will notice the hight restrictions and use an alternative route by themselves. Some drivers still do not think that the warnings are applicable to them and proceed to crash into the bridges after the lights turn to green.[34]

In United Kingdom, a more advanced system that targets a specific vehicle was deployed at Blackwall Tunnel. An advanced overheight detection system is installed 1.6-kilometre (0.99 mi) ahead of the tunnel. It is equipped with automatic number-plate recognition that reads off the plate number and sends that over to variable-message signs to instruct the vehicle with that particular plate number to divert to the provided alternative route. The system reduced tunnel strikes by 38%. In Australia, a water curtain falling on all traffic lanes with a laser projection of a large stop sign in front of the Sydney Harbour Tunnel is used as the last resort to stop overheight vehicles that ignore all the warning sings and traffic lights.[35]

There are several systems that can be equipped in trucks or other commercial vehicles to provide bridge strike prevention. These include specialized truck GPS systems that have clearance information, systems that measure the actual vehicle heights, and onboard sensors that detect upcoming low-clearance structure. More advanced onboard systems can electronically receive warnings from overheight vehicle detection systems. They can apply a brake if drivers fail to react.[33]