Geospatial scientists have developed a new program to monitor street signs needing replacement or repair using Google Street View


Geospatial scientists have developed a new program to monitor street signs needing replacement or repair by tapping into Google Street View images.

The fully-automated system is trained using AI-powered object detection to identify street signs in the freely available images.

Municipal authorities currently spend large amounts of time and money monitoring and recording the geolocation of traffic infrastructure manually, a task which also exposes workers to unnecessary traffic risks.

Results just published in the journal of Computers, Environment and Urban Systems show the system detects signs with near 96% accuracy, identifies their type with near 98% accuracy and can record their precise geolocation from the 2-D images.

Study lead author and RMIT University Geospatial Science Honours student, Andrew Campbell, said the proof-of-concept model was trained to see ‘stop’ and ‘give way'(yield) signs, but could be trained to identify many other inputs and was easily scalable for use by local governments and traffic authorities.

“(Municipal authorities) have requirements to monitor this infrastructure but currently no cheap or efficient way to do so,” Campbell said.

“By using free and open source tools, we’ve now developed a fully automated system for doing that job, and doing it more accurately.”

The team found during investigations that mandatory GPS location data in existing street sign databases was often inaccurate, sometimes up to 10m off.

“Tracking these signs manually by people who may not be trained geoscientists introduces human error into the database.

Our system, once set up, can be used by any spatial analyst—you just tell the system which area you want to monitor and it looks after it for you,” Campbell said.

The system identifies and locates the stop signs. Credit: RMIT University

Campbell credited the project’s initial concept to his industry mentor at Alpine Shire Council and RMIT Geospatial Science alumnus, Barrett Higman.

RMIT geospatial scientist and project co-lead, Dr. Chayn Sun, said the fact that some councils were already attaching cameras onto rubbish trucks to gather street footage showed how valuable visual data were becoming, given what technology could now do with it.

“This imagery is critical for local governments in monitoring and managing assets and with the huge amount of geospatial applications flourishing, this information will only become more valuable,” Sun said.

“Ours is one of several early applications for this to meet a specific industry need but a whole lot more will emerge in coming years.”

Sun said footage from other sources, like that from rubbish truck cameras or any other geo-referenced imagery of the road network collected by municipal authorities, could also be fed into the system.

“Where footage is already being gathered, our research can provide councils with an economical tool to drive insights and data from this existing resource,” she said.

Traffic signs are critical elements of the highway because they communicate the rules, warnings, guidance, and other highway agency information that drivers need to safely and efficiently navigate roads and streets. Well maintained signs are important as they help drivers make good decisions.

This guide is intended to help local agency maintenance workers ensure their signs are maintained to meet this need.

This guide is not a comprehensive design guide for roadway signing—there are many aspects to signing that cannot be covered here. For standards and guidance on all signs, refer to the Manual on Uniform Traffic Control Devices (MUTCD), the Standard Highway Signs Handbook, and the many other references found at the end of this guide. These references provide more detailed information on the sign topics briefly covered here:

  • Principles and Types.
  • Materials.
  • Supports.
  • Installation.
  • Management System.
  • Inventory.
  • Inspection.
  • Preventive Maintenance.
  • Repair and Replacement.
  • Recordkeeping.

Appendices are provided for:

A. Materials and equipment used for sign maintenance.
B. Clear zone description.
C. Typical work zone traffic control plans for sign maintenance activities.


Traffic Sign Principles

While there is much to be said about the design, placement, operation, and maintenance of signs (see, for example, The Traffic Signing Handbook), there are a few key principles that are critical for traffic signs to be effective. As with any traffic control device, a traffic sign should meet five basic requirements stipulated in the MUTCD, namely:

  • Fulfill a need;
  • Command attention;
  • Convey a clear, simple meaning;
  • Command respect from road users, and
  • Give adequate time for proper response.

Even during maintenance operations, it is important for staff to question every sign that they work on to see if it is needed and meets standards applicable to its use. The decision to use a sign at a particular location should be made on the basis of either an engineering study or the application of engineering judgment. Local jurisdictions that do not have engineering staff knowledgeable of the MUTCD should seek assistance from their State transportation agency, another jurisdiction, or traffic engineering consultant. The Local Technical Assistance Program (LTAP) in each State and tribal area can be a valuable resource for this assistance as well.

Traffic Sign Types

The MUTCD specifies three classes of signs defined by their function:

  • Regulatory signs.
  • Warning signs.
  • Guides signs.

Regulatory Signs

The rules of the road—the laws and regulations—are often communicated through regulatory signs. These are important signs to maintain because the absence of, or damage to, a regulatory sign could result in or contribute to a severe crash.

Regulatory sign: STOP sign
Regulatory sign: YIELD sign
Regulatory sign: SPEED LIMIT 50 sign

Regulatory signs are usually a combination of black, white and red colors as shown by these examples.

Damaged or missing regulatory signs (especially the STOP, YIELD, ONE WAY, and DO NOT ENTER signs) should be replaced or repaired within hours of the agency having notice of them missing, down, or damaged.

Warning Signs

Warning signs are important because they provide drivers with advance notice of potentially hazardous situations or conditions of the road ahead that may not be apparent. They are particularly helpful to motorists who are unfamiliar with a particular road.

Warning sign: Turn Ahead sign
Warning sign: Signal Ahead sign
Warning sign: ONE LAND BRIDGE sign

Warning signs, such as these, have a yellow background.

Damaged or missing warning signs should be repaired or replaced as soon as possible. As a general rule, deficiencies should be corrected within three days of notice.

Guide Signs

Guide signs are used for driver navigation—to help a driver to get to a particular destination.

There are a variety of guide signs that display route designations, destinations, directions, distances, services, points of interest, and other geographical, recreational, and cultural information.

The absence of these signs could cause motorists to miss their route and destination, resulting in less than efficient navigation.

It can also lead to erratic maneuvers, such as slowing or stopping in the roadway and making abrupt turns.

For local roads and streets, common guide signs include: street name signs, route signs, and destination and distance signs.

General service signs, e.g. the hospital symbol, and general information signs, e.g. the library symbol, are included within the guide sign group.

Guide sign: Destination sign

Destination sign

Guide sign: Street Name sign

Street name sign

Guide sign: Hospital sign

Hospital sign

Guide signs are usually white legend on a green background, but can be other backgrounds, such as brown or blue.

Replacement of damaged or missing guide signs should be done within approximately seven working days from notice, recognizing that timing of the replacement is dependent upon how critical the sign is. For example, a street name sign is essential for emergency responders to find streets and locations on those streets. Also, since many guide signs have to be specially fabricated due to the variation in the sign message, it may take a few days before they can be assembled or purchased.

Signs in Work Zones

Whenever there is construction, utility work, or maintenance being done on the road or roadside, temporary traffic control is necessary, which usually includes temporary warning signs. The warning signs used for this purpose have a black legend and border on orange background. While these are temporary signs, it is important that they be maintained because travel through work zones can be hazardous due to unusual or different conditions that drivers may not expect. These signs should not be allowed to deteriorate much below their original condition. For more information on this topic refer to the MUTCD, Part 6. Appendix C provides three typical traffic control applications from the MUTCD that would likely be used for sign maintenance activities.

Warning sign: ROAD WORK sign
Warning sign: DETOUR sign
Warning sign: Flagman Ahead sign

In most cases warning signs such as these have an orange background. (Note: for the signs on left and center, the word AHEAD or the distance [e.g., 500 FT] would fit within the dashed lines.)

The response time frames for correction of missing or damaged signs mentioned above are provided to illustrate relative priority in providing sign maintenance activities. Each agency should adopt a policy that addresses sign repair response procedures and times both for normal work periods as well as after hour needs.


A basic understanding of the materials that make up a sign is needed to carry out a good maintenance program. This section provides a brief overview of the materials for the sign face and the sign backing.

Sign Face Sheeting

The sign face consists of film material, called sheeting, which is comprised of glass beads or microprisms to provide visibility at night. Highway signs, that are not illuminated by external lights, are visible at night because they have sheeting made of retroreflective material. Retro-reflection is a type of reflection that redirects incident light from the sign face back to the source—the vehicle headlights.

This diagram shows how light travels through a micro-prism.
This diagram shows how light travels through a glass bead. In both diagrams, incident light goes in and is reflected back out.

Retroreflection is achieved through either glass beads or microprisms imbedded in the sheeting.

Over time manufacturers have developed different types of sheeting to accomplish retroreflection, initially using small glass beads, but now more commonly using microprisms with various angular designs. Sheeting manufacturers have specific brand names for their materials. ASTM International has classified the different types by conformance to the retroreflectance properties, color, and durability. The types for rigid surface signs are as follows:

  • Type I — commonly referred to as engineering grade and typically made as an enclosed lens, glass beads sheeting.
  • Type II — commonly referred to as super-engineering grade and also typically made of glass beads.
  • Type III — commonly referred to as high intensity and can be made of encapsulated glass beads or microprisms.
  • Types IV, VIII, IX and XI — no common identifiers but referred to as prismatic with the differences related to the varying angles of the microprisms.

ASTM has established new sheeting specifications for minimum retroreflectivity values, called ‘coefficient of retroreflection’ (RA) for each sheeting type. RA is a quantitative measure and is expressed in metric units of candelas per lux per square meter (cd/lx/m2).

The specification provides an initial RA value for a prescribed entrance angle and observation angle—common angles within an instrument that measures RA and relates somewhat to roadway/vehicle geometries.

Minimum RAs for the various sheeting types of white, yellow, fluorescent yellow, green and red color sheeting used for permanent rigid signs at the common angle readings of -4 degree entrance angle and 0.2 degree observation angle—angles typically set for measuring RA in a portable instrument (see pg. 27) are shown in Table 1.

Table 1. Minimum Coefficient of Retroreflectivity (RA) for Several Types of New Sheeting Used for Rigid Signs

 Coefficient of Retroreflectivity for 0.2 deg Observation 
and -4 deg Entrance Angles
Sheeting TypeWhiteYellowFlourescent YellowGreenRed

Source: ASTM Standard Specification D4956-09

The values shown in Table 1 are for newly purchased sign sheeting. Exposed to the sun and weather over time, the colors will fade and the retroreflective qualities of the sign sheeting will deteriorate to such an extent that the sign will no longer be readable, both day and night, at a distance needed by the driver.

The 2009 MUTCDincludes a requirement for minimum maintained retroreflectivity levels for most traffic signs. These values are shown in Table 2 for certain types of permanent signs most commonly found on local, non-freeway roads.

Table 2. Minimum Maintained Retroreflectivity Levels for Specified Signs.

GuideWhite on Green: White
(Ground Mount Only)
Do Not Use= 120= 120
GuideWhite on Green: Green
(Ground Mount Only)
= 7= 15= 15
WarningYellow: = 48 in. or Bold SymbolDo Not Use= 50= 50
WarningYellow: < 48 in.Do Not Use= 75= 75
RegulatoryWhite on Red*: White= 35= 35= 35
RegulatoryWhite on Red*: Red= 7= 7= 7
RegulatoryBlack on White: White= 50= 50= 50

* White to red contrast ratio shall be at least 3:1.
Source: Modified from MUTCD, 2009

Based on the values in Table 2, Type I sheeting material should not be used for warning signs and for the white legend on green background guide signs because it does not provide the retroreflectance level required by the driver. The higher level sheeting types will provide a brighter sign and in general have a longer service life, allowing for an overall cost savings.

Agencies will have until January 2012 to establish and implement a sign assessment or management method in compliance with the 2009 MUTCD to maintain minimum levels of retroreflectivity. The compliance date for replacing signs that do not meet minimum requirements is January 2015, except for street name signs, which is January 2018.

To learn more about sign materials and especially sign retroreflectivity, visit the following FHWA web site:

Sign Backing

The retroreflective sheeting material is applied to the rigid sign backing (also known as the substrate), which can be aluminum, wood, or a composite plastic.

Aluminum is by far the most commonly used material.

The aluminum backing can be reused often, reducing the cost of signing and can be recycled easily to recoup a small portion of the cost.

Plywood is used occasionally for temporary signs because it can be cheaper and is lighter than aluminum. Fiberglass-reinforced plastic materials are used infrequently because of their handling and recycling issues.

Photo. The rear of a sign backed with aluminum is shown.

Most agencies use aluminum for sign backing.


Any device that is used to display the traffic sign is known as the sign support, which consists of the vertical post and, if needed, any stiffeners onto which the sign panel is attached. Most posts for roadside post-mounted signs are one of the following types:

  • Wood post.
  • U-channel steel post.
  • Square or round tube steel post.
  • I-beam steel post.

A sign support can become a deadly hazard when struck by a vehicle that drives onto the roadside; therefore, there is an MUTCD standard that requires all roadside sign supports in the clear zone (see Appendix B for description of clear zone) to be breakaway, yielding, or shielded by a barrier or crash cushion. ‘Breakaway’ is a term to describe crash tested sign supports that break or bend upon impact.

This includes sign supports that, when struck by a vehicle, separate from the base and are knocked ahead of or up and over the errant vehicle.

A ‘yielding’ support refers to a support that bends, allowing a vehicle to run over it. Barriers are typically not installed to shield roadside signs, especially on local roads; hence, the design of the post, regardless of the type, must provide the breakaway or yielding feature.

All sign supports on highways within the clear zone must either be of a breakaway type meeting the crashworthiness criteria of NCHRP 350 (see references) or be shielded by guardrail, barrier, or an energy absorbing system meeting NCHRP Report 350 or the AASHTO Manual for Assessing Hardware (MASH) criteria. Additional information on acceptable breakaway sign supports can be found at FHWA’s Office of Safety website:

Wood Post

Wood posts are frequently used, especially in regions where wood is economical compared to metal type supports. These posts usually come in sizes of 4 x 4 inches to 6 x 8 inches. All posts above 4 x 4 inch nominal size must be drilled perpendicular to traffic flow to allow the post to break away if struck by a motor vehicle.

Small supports have a cross section of approximately 16 square inches. This type of post should be buried about 30 to 36 inches deep depending upon the type of soil and backfill; check State specifications or with LTAP for embedment depth and backfill material. The post may need to be buried even deeper to reduce vandalism and reduce dislodging by heavy winds. One or two posts may be used to make up the sign support, but adding posts too close together can affect crashworthiness.

Photo. A small wood sign support.

Small wood support is typically a 4 x 4 inch post.

Large supports should be drilled to provide the breakaway characteristic. For example, a 6 x 8 inch wood post can be used if the cross section is weakened by drilling two 3-inch holes (drill perpendicular to roadway). A 4 x 4 inch wood post is the largest undrilled wood post recommended to act as a breakaway support.

A large wood sign post with two holes drilled in it at 4 and 14 inches above the ground.

Holes are needed for wood posts greater than 4 x 4 inches.

U-Channel Steel Post

The U-channel, hot rolled steel post is another common small sign support. It is considered breakaway since it will bend, break or pull out of the ground when it is hit.

Post Support. The post should be driven into the ground and not encased in concrete. A broken or damaged post is easier to remove if it is not driven or set into the ground more than 3.5 feet.

Photo. A typical u-channel post.

Typical u-channel post.

Breakaway Devices. A U-channel post of re-rolled rail steel weighing 3 pounds-per-foot or less meets breakaway requirements by itself. If a heavier post is used, splices can be purchased commercially to install at ground level; alternatively a stub post of the same material can be set in a concrete base with a 4-inch length available to bolt to the sign post as a base connection. These devices improve safety when the post is hit, will make repair easier, and will make it possible to use a U-channel post when it has to be placed in a concrete area.

Photo and diagram. The photo depicts a breakaway treatment for a u-channel post.
Photo and diagram. The diagram is of the same, showing the base below the ground, the post, and the 4-inch overlap between the two.

Breakaway treatments for u-channel posts.

Square Steel Tube

Another sign post is the square steel tube (perforated) design, which is used in many localities. Posts of this type are also considered breakaway if they are 2 ¼ inches or less in size.

Post Support. Posts can be driven into the ground. Do not place concrete around the post. A broken or damaged post is easier to remove if it is not driven or set into the ground more than three feet.

Photo. A typical square steel post is shown.

Typical square steel post.

Breakaway Devices. As with the U-channel post, sleeve assemblies can be used for the base or slip couplings can be used near the base. These devices will increase the safety of a sign if it is hit and make it easier to repair. After the sign has been hit, the broken stub of the post can be removed from the base sleeve and a new sign post put back in place.

Photo of example breakaway square steel posts is shown. This is a photo of a sleeve assembly.
Photo of example breakaway square steel posts is shown. This is a photo of a slip coupling.

Two types of breakaway designs.

I-Beam Steel Post

This post type is used when it is necessary to support large sign panels, which is common for roadside post-mounted guide signs.

All large steel posts use a breakaway feature, unless protected by barrier or placed out of the clear zone. This is usually accomplished by using a slip base that connects the post to the foundation. When struck, the post slips off the foundation at the bottom, and rotates around the hinge plate below the sign panel. This allows the vehicle to safely pass under the sign after impact.

This photo depicts a large sign supported by I-beam steel posts.
This photos depicts a close up of the base of the posts.

I-beam steel posts with a slip base are needed for larger roadside guide signs.

There are many other products available commercially for sign supports. Use depends on local requirements and costs. This publication only shows four of the most common types of small sign supports. The comprehensive guide to breakaway sign supports is A Guide to Small Sign Support Hardware.


Location and Height of Signs

When installing a sign for the first time or as a replacement to an existing sign, the sign technician should be aware of the requirements for its position on the roadside. Signs are positioned longitudinally along the road, laterally from the edge of the road, and vertically above the ground. If a sign replacement is warranted, the sign technician should not assume the original sign was placed correctly. It is important to check the location of the support and the mounting height of the sign above the roadway to ensure the sign is visible and crashworthy.

The longitudinal location of the sign is dictated by the sign type and the specific situation. There are too many different situations to cover in this guide, so refer to the current edition of the MUTCD, your State standards and/or directives, and your agency’s requirements.

The figures on the next page represent the recommended height and lateral location as provided in the MUTCD.


Diagram. Figure 2A-2 from the 2009 MUTCD is shown. It gives 8 examples of heights and lateral locations of signs for typical installations.

Source: MUTCD, 2009

Positioning Signs

To obtain maximum retroreflection from traffic signs, yet eliminate specular glare, signs should be correctly aligned. Specular glare is the mirror type reflection characteristic of any glossy surface. Under severe circumstances, such as direct sunlight, specular glare can make the sign legend unreadable. Often this becomes apparent from driver complaints. To avoid the occurrence of specular glare, signs should be positioned slightly more than perpendicular to the roadway (93 degrees is recommended by sheeting manufacturers).

Photos. A sharply curving street is shown during the day and at night.
The road features four chevrons, 2 of which are out of alignment, making them difficult to see during the day photo and nearly impossible to see in the night photo.

When signs are out of proper alignment, they will not be as visible at night.

More information: Andrew Campbell et al, Detecting and mapping traffic signs from Google Street View images using deep learning and GIS, Computers, Environment and Urban Systems (2019). DOI: 10.1016/j.compenvurbsys.2019.101350

Provided by RMIT University


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