Center for Problem-Oriented Policing

Responses to the Problem of Graffiti

Your analysis of your local problem should give you a better understanding of the factors contributing to it. Once you have analyzed your local problem and established a baseline for measuring effectiveness, you should consider possible responses to address the problem.

The following response strategies provide a foundation of ideas for addressing your particular problem. These strategies are drawn from a variety of research studies and police reports. Several of these strategies may apply to your community's problem. It is critical that you tailor responses to local circumstances, and that you can justify each response based on reliable analysis. In most cases, an effective strategy will involve implementing several different responses. Law enforcement responses alone are seldom effective in reducing or solving the problem. Do not limit yourself to considering what police can do: give careful consideration to who else in your community shares responsibility for the problem and can help police better respond to it.

Graffiti is not solely a police problem. The police role should be one of support and assistance. Effective responses to graffiti may combine management practices, design and maintenance, and involve the general public, individual victims, criminal justice officials such as prosecutors and judges, and others. Responses to graffiti should be comprehensive and coordinated, while costs and available resources should be carefully evaluated.

Responses to graffiti must be thorough and consistent, as some offenders may be highly opportunistic, adaptive and tenacious. Responses should include ways to monitor graffiti and address changes in time, location and methods of applying it.

Reducing Rewards to Offenders

Rapid identification and removal of graffiti has been shown to reduce its occurrence. This approach directly addresses the motives of many offenders by reducing the notoriety associated with graffiti's visibility. The two-step process involves routine monitoring to quickly spot graffiti, and rapid removal of the graffiti. In New York's successful approach to transit graffiti, it was initially removed within two hours of identification.†† In St. Petersburg, Fla., business owners are required to remove graffiti within 48 hours.16

† This "law of diminishing vandalism" is that persistence in cleaning up pays off. See SloanHowitt and Kelling (1990); Scott (1989); Cheetham (1994); Clarke (1978) [PDF]; and Governing (1994).

†† If graffiti cannot be removed quickly, trains are taken out of service. For train stations, graffiti is removed within 72 hours. Similar quick cleanups have occurred in Philadelphia (Scott 1989). In London, graffiti is cleaned from large stations within 24 hours.

  1. Detecting graffiti rapidly and routinely. There are two primary ways to gather information about the incidence and location of graffiti: systematic monitoring of graffiti-prone locations, and increased reporting. Both are used to rapidly detect graffiti incidents; document the location and time of occurrences, and content of graffiti; and to trigger responses.
    • Monitoring graffiti-prone locations routinely. Quick detection of graffiti provides better information for developing effective interventions. A graffiti database can be used to track incidents and illuminate patterns, identify chronic offenders and/or interpret gang activities or plans encoded in graffiti. Monitoring may include documenting graffiti through photographs or video. In some places, graffiti provides a barometer of gang activity and relations between gangs. To monitor graffiti-prone locations, Phoenix has used night vision and digital cameras, while Philadelphia and Sydney have used closed-circuit television (CCTV). In Philadelphia and on Los Angeles buses, plainclothes officers have monitored graffiti. In other jurisdictions, Neighborhood Watch and other groups systematically monitor graffiti. In Lakewood, Colo., citizens' academy graduates take graffiti reports, photograph graffiti and monitor graffiti locations. In New South Wales, "graffiti spotters" have this role. Employees such as bus drivers or maintenance workers can immediately report vandalism through two-way radio.
    • Increasing reporting of graffiti and offenders. Anonymous graffiti hotlines, some operating 24 hours a day, collect information about graffiti incidents. Communities have also used cell phone reporting, voice mail, emergency cell service, and connection to neighborhood watch groups.

      † Police usually encourage citizens to call 911 regarding graffiti in progress; they discourage citizens from confronting offenders. Citizens can report graffiti not in progress to hotlines.

      Some jurisdictions pay graffiti reporters' cell phone charges. In London, people can use free telephones in transit stations to report offenses. In other jurisdictions, transit riders are encouraged to report graffiti and offenders. Numerous jurisdictions offer a cash reward of $200 to $1,000 if a tip leads to a conviction.

      In some jurisdictions, graffiti reports may be suppressed due to concerns about retaliation by gang members or taggers. Widespread public participation in both open and anonymous reporting usually addresses these concerns, but police should be aware of this potential problem.

Addressing Transit Graffiti in New York City

The experiences of the New York City Transit System illustrate varying approaches to graffiti. Graffiti began to appear on subway trains in the 1960s; by 1970, it was a huge problem. The public was fearful, and ridership on trains declined.

The motive for the graffiti was "getting up" and getting noticed; there were no indications the graffiti was gang-related. Instead, the graffitists or taggers sought to build their reputation through the sheer quantity of their graffiti. As competition among them increased, they distinguished themselves through writing style, embellishment, graffiti size, and location — either in unusual spots or in previously unmarked spots. One prolific vandal produced 10,000 graffiti markings.

Despite the severity of its ongoing fiscal crisis, New York City adopted a variety of anti-graffiti strategies in the 1970s: punishing offenders by making them clean up trains marked with graffiti; using fencing with razor wires to protect the vast train yards; and developing materials to ease graffiti removal, materials that were later found to be environmentally hazardous. The methods all failed to substantially reduce the amount of graffiti.

In 1984, the city adopted a system to monitor trains and clean those marked with graffiti within two hours; otherwise, they took the cars out of service. They also began to store clean trains in highly secure yards that featured 24-hour-a-day work crews, enhanced lighting, routine fence maintenance, and undercover police. The initiative focused on the most problematic times, locations and train lines; initially, all trains were monitored, but random checks were later successfully used to maintain clean trains. In addition, repeat offenders were targeted for parental contact and enhanced penalties.

In contrast to the earlier initiatives, this anti-graffiti effort began with a handful of trains (those detected with graffiti) and built up to cover the entire system. Importantly, rather than focusing on using the criminal justice system, this approach addressed the offenders' underlying motives. Immediately removing graffiti-marked trains from service severely limited the vandals' exposure.

  1. Removing graffiti rapidly. One of the most promising responses to graffiti is consistently getting rid of it, and doing so quickly. The removal process may vary substantially depending on the type of graffiti tool and the type of material vandalized. Many of the methods are timeconsuming and can be quite expensive, so a jurisdiction must be able to tap sufficient resources to fully implement this approach. Some types of cleanup — including paint-overs — may be affected by cold or wet weather. Removal may be timetargeted, such as during predawn hours, to further reduce exposure. Rapid removal is key, and many jurisdictions try to remove graffiti within 24 to 48 hours; in some obscure locations, such as drainage ditches, graffiti may be removed less quickly.

    † The type of surface graffiti is placed on is a major factor because graffiti-removal products may damage some surfaces. The type of marking agent is also a factor: some paints are reversible. There is a wide range of graffiti removal products available, including chemical sprays, aerosols, gels, and poultices. Cleaners are either alkaline or acidic; the latter can damage masonry, and neutralizing techniques must be incorporated when using either. Physical removal methods include low- and high-pressure water cleaning, often with detergents, and sandblasting. Physical removal is more expensive, and is typically used for large areas where other methods have failed.

    There are four major types of removal or cover-ups:
    • Painting over graffiti. Painting over graffiti appears to be the most common and relatively cheapest method of removing it. Although paint-overs can be expensive if recurring, the approach is widely accessible, and usually requires no special skills or technology. Some cities provide recycled paints for free; some cities have cleanups funded by contributions; and in some cities, businesses donate paint. Property owners victimized by graffiti offenders often supply their own paint. They can match chips of paint at home supply stores. Once they make a paint match, they should keep a supply of the paint readily available. In areas with heavy graffiti, property owners can unify colors (e.g., of alley walls and fences) to make routine paint-overs easier. Painting over graffiti may require the use of a sealer to prevent bleeding through.
    • Removing graffiti chemically. There are a variety of chemical removal products available, but care should be taken in selecting one. The use of some removal products on certain porous surfaces may create a shadow of the graffiti. Paint companies sometimes donate paint-removal supplies.
    • Cleaning graffiti off. Depending on the surface and marking agent, many surfaces can be cleaned of graffiti. Methods include sandblasting with high-pressure hot-water jets — and sometimes baking soda — to remove graffiti from cement and other unpainted surfaces, although this, too, can be expensive and leave a shadow. Lasers to remove graffiti are becoming available.
    • Replacing signs, materials and other items vandalized. Replacement is appropriate for materials from which graffiti cannot be painted over, chemically removed or cleaned.

      The source of labor for removing graffiti may vary. Cleanup squads may consist of volunteers, employees or adjudicated offenders. Graffiti removal may be coercive. A large number of jurisdictions hold the property owner responsible for graffiti removal. Sanctioning victims requires that they clean graffiti up quickly or get fined.17 Citizens may get paint or physical assistance from volunteers, if needed. Cities can use nuisance ordinances, zoning codes or graffiti ordinances to force owners to clean up quickly, which may be necessary for absentee owners. Alternatively, some cities clean up graffiti and then bill the owner. Some cities do the first cleanup for free; the owner then has responsibility for subsequent cleanups.

      Numerous jurisdictions use graffiti removal as a court-ordered sanction for offenders and other misdemeanants. In some jurisdictions, such sanctions require victim restitution, reflecting a restorative justice approach.

Increasing the Risk of Detection

Because graffiti offenders usually operate in darkness, where there is little chance of being seen, few are apprehended. Increasing the likelihood of their being detected increases the risk of apprehension.

  1. Increasing natural observation of graffiti-prone locations. The likelihood of detecting offenders can be increased by installing, upgrading or maintaining lighting. (While most offenders operate in the dark, additional lighting may actually attract graffiti in some isolated or remote locations. An alternative is to install motion-activated lighting, which may signal unauthorized property use.) In addition, shrubbery or trees that conceal areas can be removed. Sight lines can be improved where vision is obscured in other ways. Other methods to increase observation involve design, such as eliminating blind spots of underpasses, or park paths, installing windows or building parking lots within view of residences and designing spacious areas with good visibility.
  2. Increasing formal observation of graffiti-prone locations. Observation of graffiti-prone locations can be improved systematically through use of police, security personnel, Neighborhood Watch, and employees with other primary duties (such as bus drivers, ticket agents, newsstand staff, lobby concierges, and on-site/residential property managers). Such observation may include the use of uniformed or undercover personnel or covert surveillance, and may target fixed locations or mobile locations such as buses and trains.
  3. Increasing electronic security. Formal observation of graffiti-prone locations can be carried out via electronic methods. CCTV has shown promising evidence of reducing vandalism, including graffiti.18 CCTV is widely used to deter potential offenders, apprehend offenders in the act or after the fact, and provide evidence in prosecutions. There are substantial up-front and operating costs to CCTV, and decisions must be made as to whether cameras will be actively or passively monitored, or activated by motion detectors. If CCTV is to be used for evidence, good picture quality, adequate lighting and follow-up investigation are necessary. If CCTV is to be used to apprehend offenders in progress, it must be actively monitored. Signs warning of CCTV are often posted to discourage offenders; such deterrence may contribute to graffiti's spread to other locations. There is also evidence that CCTV's crime prevention benefits may spread to other locations. CCTV will not be effective everywhere, but can be adapted. For example, video surveillance with infrared technology has been used on buses, while electronic surveillance robots monitor CCTV screens in some jurisdictions, and emit warning alarms. Portable CCTV can also be used, and dummy CCTV has been effectively used to supplement the real thing. Other types of electronic security include infrared beams, which are used around trains in London. Use of CCTV may result in reduced vigilance, as electronic surveillance may create a false sense of security. But the presence of CCTV may also reassure citizens, and public support for it is often high.
  4. Conducting publicity campaigns. On their own, publicity campaigns are of limited effectiveness. However, many publicity efforts are combined with other strategies. A number of publicity campaigns can be described as beautification efforts, consisting of community cleanup days to eliminate graffiti, litter and other signs of disorder. In many jurisdictions, these cleanup days require volunteers, but some may involve court-adjudicated offenders who are working off community service time. In contrast to the systematic graffiti removal described above, publicity campaigns are usually onetime or episodic cleanups of specific areas. An extension of the cleanup programs are ownership initiatives such as Adopt-a-Block, Adopt-a-Bus, Adopt-aStation, or other efforts to maintain the "cared for" environment in public areas. Some of the adoption schemes involve painting murals on transit shelters, invoking a presumed conscience that deters graffiti offenders from marring others' artistic endeavors. It is assumed that graffiti is easier to detect where no other graffiti exists, and cleaned areas invoke a sense of ownership and responsibility among users of the areas. Other publicity efforts include posters to publicize anti-graffiti efforts, public service announcements, flyers, brochures, and the like. Publicity campaigns often include information on the harms of graffiti, the costs of graffiti, how to detect a graffiti offender, and how to report graffiti. This educational effort is often targeted at parents, schools, businesses, civic groups, transit system users, and/or the general public. Publicity and educational campaigns have been shown to be effective in reducing graffiti when used to publicize surveillance of vandalized buses; the effects even extended beyond the crime prevention targets.19 Publicity campaigns often discourage the use of graffiti in advertising and art exhibits, as well as media coverage of graffiti, recognizing that such attention serves to further contribute to the notoriety graffiti offenders seek. Care is taken to avoid glorifying graffiti, and generating more of it as a result.

Increasing the Difficulty of Offending

  1. Vandal-proofing graffiti-prone locations. Graffiti offenders can be thwarted by vandal-proofing vulnerable surfaces in vulnerable areas, a process that often involves modifying surface textures. Anti-graffiti coverings and surfaces make surfaces easy to clean, difficult to write on, or both. There are six primary types:
    • Paint-like products such as polyurethane-based coatings are resistant to graffiti and easy to clean. These are suitable for steel, concrete and brickwork. Sealers on concrete prevent absorption.

      † Some of these products may produce toxic fumes in case of fire.

    • Wash-off coatings — known as sacrificial coatings — are wax or silicon applications on walls or buildings. When hot water is applied, these coatings break down, allowing graffiti to be washed off.††

      †† These coatings must be reapplied; the surface dissolves when graffiti is cleaned off.

    • Textured surfaces are not attractive targets for graffiti, as they obscure legibility. Such surfaces are particularly difficult for offenders to draw on or paint. Such surfaces include deeply grooved surfaces and rough surfaces††† such as exposed rock, rough cement and dimpled stainless steel, like that used in London telephone kiosks.

      ††† These surfaces are harder to mark, but are difficult to clean.

    • Dark or colorful surfaces make graffiti less visible, thus deterring offenders. Dark surfaces are more difficult to mark up, although light paint can be used. Colorful or busy surfaces, such as advertisements on the sides of buses, deflect graffiti.20 Flecked or spotted wall surfaces also mask graffiti.
    • Non-solid surfaces, such as open-grill storefront security screens rather than solid panels, may deflect graffiti.
    • Easily cleaned materials may be installed in highly vulnerable areas. These include vitreous-enamel panels or glazed ceramic tiles from which graffiti washes off; wired glass that can be cleaned with scrapers;†† polyester film over glass; plastic laminates, which make for easier cleaning; and signs with surfaces resistant to marker pens and spray paint.

      † These washable walls are used in larger London train stations.

      †† The alternative, polycarbonate surfaces become hazy.

    Some materials cannot be effectively protected from graffiti. Graffiti-prone surfaces can be replaced with standard-sized, inexpensive materials. These include transparent, replaceable glass or polycarbonate panels in bus shelters, and replaceable polycarbonate covering signs.

  2. Controlling access to graffiti-prone locations. Controlling access to graffiti-prone locations physically bars offenders from vulnerable areas. Means of access control include:
    • graffiti hoods to buffer freeway signs;
    • metal baffles on sign poles, which work like squirrel baffles on bird feeders;
    • walls, fences, locked alleys, barriers, chasms, and rails, sometimes supplemented by barbed wire;
    • recessed walls;
    • dense or thorny plants, or climbing vines; and
    • razor wire or jagged metal wrapped around sign poles.†††

      ††† Some of these measures impose social costs by making areas look like war zones. Access controls with forbidding appearances may be better left to isolated areas.

    In some cases, signs have been moved out of reach of vandals, while bus stops and other frequently vandalized targets have been relocated. Environmental design to limit access to graffiti surfaces can best be incorporated into planning and construction, but may also be adapted to existing structures. An example of effective environmental design is the recessed walls of the Washington, D.C., metro system; subway walls are physically separated from the public.

    Police or security patrols, guards and dogs may supplement access control. Access to residential or commercial properties may be restricted to those with resident or employee identification cards, while visitor access may be controlled through entry phones.

    Much like environmental design, situational design reduces the opportunity for graffiti. The absence of toilets, seating, fast food, and lockers in transit stations effectively discourages potential offenders from loitering. In Hong Kong, a limited life to transit tickets encourages people to quickly move through stations before their tickets expire, thus discouraging loitering. In Washington, D.C., the subway system generally closes at midnight on weekdays and somewhat later on weekends, thus limiting opportunities for vandalism. Since graffiti often takes place late at night, limiting hours reduces opportunities for vandalism at times when there are typically few other riders or employees to deter the offender or witness the offense.

  3. Focusing on chronic offenders. Approaches that focus exclusively on enforcement to control offenders have had little effect on the amount of graffiti.21 Apprehending and prosecuting graffiti offenders is difficult. Graffiti is not routinely reported to police, it is difficult to catch offenders in the act, and may be impossible to find witnesses or tangible evidence of graffiti offenses. In addition, police have competing priorities, and sanctions against offenders are often weak, consisting of community service and fines.

    Some graffiti offenders are prolific; a small group typically accounts for a large portion of all offenses. Efforts that focus on chronic offenders show promise. Chronic offenders can be identified through graffiti investigations. Since offenders tend to replicate their graffiti, it has unique characteristics, like a signature, and different incidents or tags can be linked to a single offender. Some taggers practice their tags in notebooks or take photographs to document their efforts; these may be used as evidence to link offenders to graffiti incidents.

    Some police conduct surveillance of known offenders and/or high-risk hot spots, collaborate with schools to detect offenders, and monitor chronic offenders, particularly those on probation. Police may use extensive intelligence databases to record information about graffiti content, locations and offenders. Such databases may include photographs or video of graffiti, mug shots of offenders, and maps of graffiti locations.

Responses With Limited Effectiveness

Numerous responses have been incorporated into efforts to control graffiti. Most have not been carefully evaluated, and are thus of unknown effectiveness. Any response can be effective if it increases the difficulties of offending and reduces the rewards for it. Many responses, however, are quite difficult to enforce.

  1. Controlling graffiti tools. A number of jurisdictions have tried to control the tools used for graffiti. Boston and other cities have banned the sale of large, wide-tipped markers. In addition, bans on spray paint sales to minors have been widely used in recent years. Some jurisdictions require stores to be licensed for and to limit spray paint sales, and require buyers to furnish their name and address. In some jurisdictions, juvenile possession of spray paint or large, indelible markers without supervision is a misdemeanor.

    † Chicago has had such a ban since 1980.

    Efforts have been made to reduce shoplifting of spray paint by placing stock away from exits and removing it from open displays. Instead, stock is often stored behind counters, in storerooms or in locked display cases. Some jurisdictions require stores to place markers in full view of clerks. Industry efforts have also been made to regulate graffiti tools. Spray valves can be modified, and restricted-use caps limited, so that offenders cannot change caps.†† Some jurisdictions encourage proper disposal of contractor painting materials so that graffiti offenders cannot access them.

    †† Graffiti offenders prefer interchangeable caps, allowing them to combine thick and thin lines. Wide caps or other caps from oven cleaners or spray starch are especially desirable.

    While there have been no evaluations of efforts that limit graffiti tools, enforcing local ordinances that do so can be difficult. Although restrictions on possession of supplies may provide an additional enforcement tool, graffiti offenders are rarely apprehended. In many tagging groups, one person carries the graffiti supplies, making it more difficult to obtain the evidence that may be necessary for a conviction.

  2. Channeling behavior into more acceptable activities. A lot of anti-graffiti efforts have involved designating particular areas or locations as legitimate places for graffiti.††† Graffiti walls or boards are often obtained through contributions from businesses. While artists may have to have a painting permit to participate, paint for such projects is often contributed.

    ††† Nugent (1998) describes a graffiti wall in Washington, D.C.'s Lafayette Park; Coffield (1991) notes the painting of a Southampton, England, garage.

    Similarly, some jurisdictions have commissioned murals to cover up graffiti or improve the community's appearance. These murals are often located where graffiti has posed a problem. Graffiti offenders appear to respect the artwork on such murals, but the surfaces can be protected with antigraffiti coating. Murals and walls showcase artists' work and may reduce incentives to vandalize. Similar initiatives to divert offenders have included art classes or programs for reformed offenders, some of which involve a contract or pledge not to produce further graffiti. These efforts may be effective in reducing the amount of graffiti in specific locations.

  3. Providing alternative activities and services. A variety of programs have been developed to address the needs of graffiti offenders who are bored, unsupervised or unemployed. These programs include mentoring, job training, counseling, tutoring, and family services. Many of these programs focus on building pride and self-esteem. Some help youth to leave gangs. Others provide alternative activities, such as sports.
  4. Involving youth in developing programs. Youth are often involved in anti-graffiti efforts to increase their sense of ownership. In Denmark, youth were involved in selecting the design and colors of buses and bus platforms. Officials there also engage in "alternative conflict solving," and meet monthly with youth to address hostility and improve communication with those who are disaffected. Anti-graffiti posters for publicity campaigns are designed through student competitions, and peer pressure is used to discourage graffiti.

    Some anti-graffiti programs involve educating youth about the harms and costs of graffiti. The youth-targeted message that graffiti is uncool is conveyed through subway and bus posters, and television and radio commercials. Sports figures may endorse the message to add potency to it.

    In some cases, former graffiti offenders create murals with anti-graffiti messages, give public talks, counsel other offenders, and organize graffiti cleanups.

  5. Expanding applicable laws. A wide variety of laws have been passed in cities and counties across the United States, providing police and prosecutors with additional tools to charge and punish offenders. In some cases, existing ordinances or statutes have been applied in new ways, including enforcing civil trespassing laws; applying nuisance abatement, which can force gangs to clean up graffiti; labeling gangs as unincorporated associations, to pursue criminal conspiracy charges; applying civil injunctions requiring offenders to stay away from certain areas; enforcing antiloitering ordinances; and applying sanctions that enhance dispositions or sentences for gang members. In addition, many jurisdictions routinely use criminal mischief, malicious mischief, property destruction, vandalism, and criminal trespass statutes or ordinances in charging graffiti offenders.
  6. Holding parents accountable. In some communities, efforts are made to educate parents in recognizing signs of graffiti offending. Parents are held accountable for juvenile offenders' actions, and may be sanctioned with fines, cleanup costs and even jail for failure to control or supervise their children. Structured juvenile diversion programs may involve parents in meeting conditions imposed on offenders.
  7. Increasing sanctions for offenders. Across the United States, jurisdictions have increased the sanctions against graffiti offenders. Some sanctions are targeted specifically at juveniles. For example, California suspends or defers the award of driver's licenses for one year; offenders can do community service to reduce the suspension time.

    Many jurisdictions use graffiti cleanup for community service to avoid adjudication, as a condition of probation, or as part of a disposition or sentence. Some communities have restorative justice initiatives in which face-to-face victim-offender reconciliation occurs, a contract is signed, and offenders pay restitution.

    In some jurisdictions, students are suspended or expelled from school for graffiti offenses. A large number of jurisdictions have involved courts in treating graffiti incidents seriously, systematically imposing fines, community service and even jail time on chronic offenders.

  8. Applying new technologies. A wide range of new antigraffiti technologies have not been tested, used extensively or evaluated. Some may be effective in specific settings under certain conditions.

    New anti-graffiti technologies include the following:

    • Listening devices positioned at chronic graffiti locations. The devices detect sounds such as spraying of paint cans, alerting police to offenses.
    • Motion detectors combined with sprinkler systems. Caltrans used this technology in Orange County, Calif., but offenders broke off sprinkler heads.
    • Lasers for graffiti removal.

    Since developing or purchasing new technologies may be quite costly for most jurisdictions, such responses should be carefully evaluated first. New technologies to respond to graffiti will likely continue to become available.

  9. Establishing juvenile curfews. Juvenile curfews have been widely adopted in the United States to address a variety of juvenile crime. For the most part, tenacious offenders can avoid detection, and police agencies must invest a substantial amount of effort to enforce curfews. While curfews may have some benefits in very narrowly defined situations, their contribution to graffiti reduction are unlikely to be substantial.
  10. Warning offenders. Many jurisdictions warn graffiti offenders about the costs of being apprehended. Sydney found that warnings of dire consequences do not work, and media attention glorifies and reinforces graffiti.22 Most warnings are intended to increase the perception of risk of detection and apprehension. Offenders, however, tend to accurately perceive that risks of apprehension are fairly low. Some warnings relate to increased sanctions for graffiti offenses. If offenders do not believe the risk of apprehension is high, they are unlikely to be concerned about the penalties for offending. Warnings directed at chronic offenders may be more effective than general warnings.

    † In some limited studies of bathroom graffiti (Mueller et al. 2000; Watson 1996), posting signs warning of sanctions, containing positive messages appealing to altruism, or conveying neutral messages — "Please do not write on these walls" — resulted in a decline in graffiti.