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Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 13 - Construction and Work Zones

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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 13 - Construction and Work Zones." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Overview of Work Zone Crashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-2 Procedures to Ensure Proper Arrow Panel Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-4 Caution Mode Configuration for Arrow Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-6 Changeable Message Signs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-8 Sign Legibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-10 Determining Work Zone Speed Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-12 13-1 C H A P T E R 13 Construction and Work Zones

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-2 O VE RV IE W OF W OR K Z ON E C RA SH ES In tr od uc ti on This guideline provides a framework for characterizing work zone crashe s and, by extension, provides guidelines for work zone design. It specifies the need for additional driver guidance in work zones based on the number, type and severity of crashes occurring in work zones. The typical work zone crash involves a male driver, age 25-34, who, while driving in clear weather during mi d-afternoon on a US Hi ghway or Interstate roadway, co mes upon slow or stopped traffic due to construction and crashes into another vehicle. As discussed below, inform ation provided by arrow panels, changeable me ssage signs, and work zone speed limits are critical to safe and efficient work zone operations. De si gn Gu id e lin es The table below summ arizes characteristics of work zones and—for each—observed impacts on crash severity or frequency. Work Zone Crash Characteristic Observed Impact on Crash Frequency and Severity General Work zones increase both rear-end and fixed-object crashes. Work Zone Area The activity area is the predominate location of work zone crashes (see the figure on next page) – Signs should be used to give drivers advance warning of upcom ing work zones. Interstate Roadways Many work zone crashes are on Interstate roadways – Selected positions of work zone signs should take into account roadway speeds and allow drivers to perceive and process the sign inform ation. Night Work Before/during crash analysis did not reveal large increases in crashes during night work unless lanes were closed and significant traffic queues developed ( 2 ) – Caution modes on arrow panels can be used to warn drivers of temporary lane closures. Car Following Patterns Research on time gaps between cars in work zones in Illinois revealed a safety paradox: as vehicle speeds increase, tim e gaps between cars decrease from those observed with lower speed cars even though it takes longer to stop a hi gher speed car ( 3 ) – Selected work zone speed lim its should main tain safe traffic flow and, usually, shoul d be within 10 mi/h of normal speed limits. Large Truck-involved Crashes in Work Zones Truck-involved crashes are more likely to be multivehicle crashes than other work zone crashes. Truck-involved crashes are mo re likely to cause injuries when the crash occurs in the activity area as compared to crashes that occur in the advance warning area ( 4 ) – Longer stopping/slowing distances for heavy trucks imply greater advance warning requirem ents for work zones, especially on roadways used by heavy trucks. Based Primarily on Expert Ju d g ment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data

13-3 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Di scu ssi on In discussions of work zone crashes, so me research sources provide specific inform ation about the driver behaviors related to crashes, in addition to the characteristics of overrepresented drivers (e.g., age, gender, etc). Other sources include the most frequent collision types, overrepresented types of vehicles (e.g., heavy trucks), and the involvement of other vehicles in work zone crashes. There is also inform ation that is mo re specifi c to how work zone characteristics may contribute to driver-related factors, including lighting conditions, pavem ent markings, and the presence of warning signs or cones. Other aspects that are covered include the type of work zone activity in addition to crash characteristics by work zone area (advance warning area, transition area, longitudinal buffer area, activity area, and ter mi nation area). The remaining guidelines in this chapter focus on signing and speed lim it information for work zones. Source: FHWA ( 1 ) C OM PO NE NT P AR TS OF A T EM PO RA RY T RA FFI C C ON TR OL Z ON E De si gn Is su es The rear-end crash is the predom inate type of work zone crash. The design of work zones, particularity speed control met hods and work zone speed lim its, should reduce speed variance or cause drivers to drive at the sa me speed. This does not necessarily m ean lowering the speed limit in the work zone, as a lower speed lim it does not always result in a lower speed variance ( 5 ). The study of fatal crashes in Texas work zones determined two design-related countermeasures: (1) design exits or refuge areas at regular intervals where shoulders are rem oved or no longer available for disabled vehicles and (2) use opposing lane dividers or arrow pave me nt ma rkings at sites where the travel direction of a lane is changed temporarily, such as when lanes are closed and two-way traffic is handled in the remaining open lanes ( 6 ). In general, the lack of consistency across states with respect to work zone signage is a problem that should be addressed in future research. Cr os s Re fe re nc es Caution Mode Configuration for Arrow Panels, 13-6 Sign Legibility, 13-10 Deter mi ning Work Zone Speed Lim its, 13-12 Ke y Re fe re nc es 1. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways . Wash ington, DC 2. Ullman, G.L., Finley, M.D., & Ullman, B.R. (2004). Assessing the Safety Impac ts of Active Night Work Zones in Texa s (FHWA/TX-05/0-4747-1). College Station: Texas A&M University. 3. Sun, D. and Benekohal, R.F. (2004). Analysis of car following characteristics for estim ating work z one safety. Transportation Research Board 83rd Annual Meeting Compendium of Papers [CD-ROM]. 4. Khattak, A.J. and Targa, F. (2004). Injury severity and total harm in truck-involved work zone crashes. Transportation Research Record, 1877 , 106-11. 5. Zhao, M. and Garber, N. J. (2001). Crash Characteristics at Work Zones; Final Report . Charlottesville, University of Virginia. 6. Schrock, S.D., Ullman, G.L., Cothron, A.S., Kraus, E., and Voigt, A.P. (2004). An Analysis of Fatal Work Zone Crashes in Texa s (FHWA/TX-05/0- 4028-1). College Station: Texas A&M University .

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-4 PROCEDURES TO ENSURE PROPER ARROW PANELVISIBILITY Introduction Arrow panel visibility depends on a number of factors, including the capability of the lamps in the panel, the type of roadway, the physical location of the panel, the panel’s relation to horizontal and vertical curves, ambient light, and weather. Procedures to ensure arrow panel visibility should include specifications for the arrow panel as well as field procedures to check in-service arrow panels. Design Guidelines ARROW PANEL SPECIFICATIONS RECOMMENDED PHOTOMETRIC REQUIREMENTS Minimum On-Axis Minimum Off-Axis Maximum On-Axis Time of Day Speed(mi/h) cd/lamp cd/panela cd/lamp cd/panela cd/panela Day 45 500 4000 100 800 NA Night 45 150 1200 30 240 5500 a Intensity requirements for the entire panel when displaying a left or right flashing arrow (10 lamps illuminated) Source: Wooldridge, Finley, Denholm, Mace, and Patrick (1). Note: cd = Candela: the SI base unit of luminous intensity Angularity Requirements Minimum angularity permitted for a Type C (high-speed and high-volume roads) arrow panel should be +/– 4° in horizontal plane (8° beam width) and +/– 3° in the vertical plane (6° beam width). Field Procedures Use of luminance to intensity measurements. Arrow should be oriented to be recognizable from 1500 ft even in curves (see figure below). Effect of Arrow Panels In lane closures, arrow panels produced almost-ideal lane changing patterns. In traffic diversions, arrow panels produced some unnecessary lane changing. Arrow panels had little effect on traffic operations in moving shoulder closures on freeways. Panel Luminous Intensity Field test resulted in recommendations for daytime of 4000 cd/panel as the minimum on-axis intensity and 800 cd/panel as the minimum off-axis intensity and a recommendation for nighttime of 5500 cd/panel as the maximum on-axis intensity. Flash Rate 25 to 40 flashes/min. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data Viewing Angle on Horizontal Curve (Adapted from Wooldridge et al. (1))

13-5 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Discussion Human factors studies conducted as part of this research are discussed in detail in Knapp and Pain (2). In Graham, Migletz, and Glennon (3), the effect of arrow panels was judged in three situations: (1) when a lane is closed; (2) in diversions where the traffic route is modified without a lane reduction; and (3) in shoulder work zones. The following findings were reported: In lane closures, the presence of an arrow panel produced lane changing patterns that are closer to ideal. In other words, the arrow panel encouraged drivers to leave the closed lane sooner and, consequently, fewer lane changes occurred close to the lane closure taper. In traffic diversions, arrow panels produced some unnecessary lane changing; however, the number of these lane changes was small, particularly at night and for truck traffic. Overall, the use of arrow panels for diversions was not shown to be beneficial. Arrow panels had little effect on traffic operations in moving shoulder closures on freeways. Conflicts due to slow-moving vehicles were greater when the caution-bar mode was used. No differences were detected in the effect of various arrow panel modes such as the flashing arrow or sequential chevron. The MUTCD (4) states that arrow panels should “not be used to indicate a lane shift.” Additionally, a separate arrow panel should be used for each closed lane in a multilane closure. Wooldridge et al. (1) made the following recommendations based on a field test conducted to examine requirements for panel luminance intensity: • Minimum nighttime on-axis intensity of 150 cd/lamp luminance • Minimum nighttime off-axis intensity of 30 cd/lamp luminance • Minimum daytime on-axis intensity of 500 cd/lamp luminance • Minimum daytime off-axis intensity of 100 cd/lamp luminance • If arrow panels are located on curves, orient them to be seen by a vehicle 1500 ft upstream. • Realign the arrow panel to be perpendicular to the driver’s line of sight at the distance desired for observation • Minimum daytime on-axis intensity of 4000 cd/panel, minimum daytime off-axis intensity of 800 cd/panel, and maximum nighttime on-axis intensity of 5500 cd/panel Design Issues Field conditions such as fog or a high level of ambient light (advertising signs) might impact the visibility of the arrow panel in the field. Mace, Finkle, and Pennak (5) note that the arrow panel should flash at a rate of 25 to 40 flashes per minute. Cross References Caution Mode Configuration for Arrow Panels, 13-6 Determining When to Use Decision Sight Distance, 5-8 Key References 1. Wooldridge, M.D., Finley, M., Denholm, J., Mace, D., and Patrick, B. (2001). Photometric Requirements for Arrow Panels (TX- 02/4940-1). College Station: Texas A&M University. 2. Knapp, B., and Pain, R.F. (1979). Human factors considerations in arrow-board design and operation. Transportation Research Record 703, 1-8. 3. Graham, J.L., Migletz, J., and Glennon, J.C. (1978). Guidelines for the Applications of Arrow Boards in Work Zones (FHWA-RD-79-58). Washington, DC: FHWA. 4. FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 5. Mace, D., Finkle, M., and Pennack, S. (2001). NCHRP Research Results Digest 259: Guidelines for the Effective Use and Procurement of Advanced Warning Arrow Panels. Washington, DC: Transportation Research Board.

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-6 CAUTION MODE CONFIGURATION FOR ARROW PANELS Introduction This guideline provides recommendations for how to use the arrow panel Caution Mode configuration during temporary traffic control (1). The Caution Mode configuration is arrow panel mode C and provides flashing non- directional information. The purpose of the Caution Mode configuration is to increase safety near highway work zones by providing early warning information to drivers indicating that caution is required while approaching and traveling through the work zone. Note that these displays are only intended to alert drivers and to call attention to the appropriate signs, channelization devices, or other temporary traffic control devices that provide the actual information that drivers must use to safely navigate the work zone. Design Guidelines Caution Mode Usage The MUTCD (1) states that the Caution Mode configuration should be used for the following situations: Shoulder work Blocking of the shoulder Roadside work near the shoulder Temporary closing of one lane on a two-lane, two-way roadway Caution Mode Display Although the MUTCD states that Flashing Box or Flashing Line displays should be used, Alternating or Flashing Diamond displays are recommended over other displays because they are more attention getting and less confusing. Flash rates of 25 to 40 flashes/min should be used. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below provides examples of different types of Cautionary Mode configurations for arrow panels (adapted from Saito and Turley (2). “Flashing Box” or “Flashing Four-Corner” “Dancing Diamonds” or “Alternating Diamonds” * “Flashing Diamonds” * “Bar” or “Flashing Line” Flashing Sequence 1 Flashing Sequence 2 Arrow Panel Caution Mode Configurations Guideline Recommended Configurations MUTCD 2003 Recommended Configurations * Use of this configuration may require the formal creation of an experimental project with FHWA.

13-7 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Discussion Caution Mode usage: The Caution Mode configuration should be used when directional information is not warranted (e.g., no merge is necessary), such as for shoulder work, blocking the shoulder, or roadside work near the shoulder (1). Some state DOTs also use the Caution Mode for slow-moving operations, such as street sweeping and striping (3). Note that the MUTCD (1) states that the Caution Mode is the only permissible usage of arrow panels when one lane must be closed on a two-lane, two-way street. Similarly, the Caution Mode should be used only if no lane change or merge is required. Consistent use of the Caution Mode in this situation helps drivers maintain a clear idea about how they should respond when seeing this display (and the same holds for arrow displays). If lane changes or merging is required on multi-lane roadways, then the arrow or chevron arrow-panel display must be used (1). Caution Mode display: The diamond-based Caution Mode displays recommended in this guideline are different from the MUTCD recommended displays, and they are not MUTCD-compliant. However, the primary reasons for recommending these diamond displays is that they appear to lead to no worse performance than MUTCD-compliant displays, while at the same time providing a display that drivers find easier to see, more attention getting, and less confusing. Two recent studies have compared the effects of diamond-based displays versus Flashing Box and Flashing Line displays on driver performance (2,3). Overall, Alternating Diamond displays lead to driver behavior that is not really different from that engendered by other display types in terms of lane migration, potential conflicts, and driver slowing (although diamond displays lead to a slightly greater degree of slowing with a statistically significant 2 mi/h reduction in mean speeds). These studies have also found important differences in driver opinions regarding the different display types (2,3). In particular, drivers rated the Alternating Diamond displays as easier to see, more attention getting, and less confusing than the other displays (3). Also, a Flashing Box display rated very poor in terms of prompting safe driving and was also rated as being much more likely to be ignored relative to Flashing and Alternating Diamond displays (2). These findings are consistent with earlier research and opinions among highway researchers and administrators. For example, Knapp and Pain (4) found that more than 50% of drivers misinterpreted the meaning of Flashing Line and Flashing Box displays. Similarly, there is some broader concern that the Flashing Line display can be interpreted as a malfunctioning flashing arrow, resulting in unnecessary lane changes (5). Finally, from a human factors perspective, the diamond displays should also be more salient and attention getting to drivers in potentially cluttered work zone environments because they are associated with a larger change of luminance (more lamps are illuminated). Design Issues There are no data currently available to suggest that either the flashing version or alternating version of the diamond displays is superior. Flashing rate should be 25 to 40 flashes per minute. Cross References Procedures to Ensure Proper Arrow Panel Visibility, 13-4 Key References 1. FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 2. Saito, M., and Turley, B.M. (2002). Dancing Diamonds in Highway Work Zones: An Evaluation of Arrow-Panel Caution Displays (UT-02.13, Final Report). Provo, UT: Brigham Young University. 3. Griffith, A., and Lynde, M. (2001). Evaluation of Arrow Panel Displays for Temporary Work Zones; Final Report (FHWA-OR-RD-02-02). Salem: Oregon Department of Transportation. 4. Knapp, B., and Pain, R.F. (1979). Human factors considerations in arrow-board design and operation. Transportation Research Record 703, 1-8. 5. Noel, E.C., Sabra, Z.A., and Dudek, C.L. (1989). Work Zone Traffic Management Synthesis: Selection and Application of Flashing Arrow Panels (FHWA-TS-89-034). McLean, VA: FHWA. http://www.fhwa.dot.gov/tfhrc/safety/pubs/89034/89034.pdf

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-8 CHANGEABLE MESSAGE SIGNS Introduction Changeable message signs (CMSs) are electronic, reconfigurable signs placed above or near the roadway. They are used to inform motorists of specific conditions or situations. CMSs must communicate messages clearly in a brief period of time. Improper CMS usage defeats its credibility and can cause motorist confusion. Display messages ideally should be limited to a maximum of two phases. Many three-phase messages can be reduced to two or one phase by eliminating unnecessary wording. Other issues to avoid include splitting information across phases, using multiple formats of calendar dates, and displaying out-of-date information. Design Guidelines Fundamental human factors, identified mostly in Dudek (1), govern the use of CMSs. Some factors that should be considered follow. Message Length and Format Words should be simple and messages standardized. Abbreviations should be used only when easily understood. The “Units” Rule For road speeds > 35 mi/h, use a maximum of four units (one unit = one answer to one question). For examples, see revised message below. For road speeds 35 mi/h use a maximum of five units. Device Consideration CMSs should be placed so that approaching drivers see them 1500 ft or more upstream, and they are not overpowered by competing road or advertising signs or conditions. Maximum Number of Words Eight for 55 mi/h roads and seven for 65 mi/h roads. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data Examples of how to revise a message to reduce reading time. Message Element Original Message Revised Message Incident on Same Freeway as CMS Location Incident Descriptor Location Lanes Affected MAJOR ACCIDENT PAST I-80 ALL LANES BLOCKED FREEWAY BLOCKED (Unit 1) PAST I-80 (Unit 2) Incident on Freeway Other than CMS Location Incident Descriptor Location Lanes Affected MAJOR ACCIDENT ON I-76 WEST AT WALT WHITMAN BRIDGE ALL LANES BLOCKED I-76 WEST BLOCKED AT WALT WHITMAN BRIDGE

13-9 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Discussion Message length and format: Because of the limited space on CMSs, suggestions for use are as follows: Messages should abbreviate the month in conjunction with the date. When future work will span days, the month should be noted only once in the message. Other factors to consider include the following: Attempts to present day, date, and time information about upcoming roadwork appear to approach the limit of driver information processing. Regardless of the format used, only about two-thirds to three-quarters of the drivers viewing the portable changeable message sign (PCMS) will be able to correctly tell whether the work activity will affect their trip (2). The “Units” rule: One unit of information equals one answer for one question. Research and operational experience indicate that no more than four units of information should be in a CMS when the traffic operating speeds are 35 mi/h or more. No more than five units of information should be displayed when the operating speeds are less than 35 mi/h. In addition, no more than three units of information should be displayed on a single message frame (1). Because motorists can process only a limited amount of information at a given time, legibility and distance must be kept in mind. Based on the known legibility distance of CMSs, the calculated maximum message length that can be read by motorists is eight words for a traveling speed of 55 mi/h and seven words for a speed of 65 mi/h. A driver traveling at 60 mi/h is moving at 88 ft/s and can see a CMS for only 7.4 s at that speed (generally a CMS is legible for 650 ft) (1). Device consideration: ITE’s proposed equipment standard states that each PCMS unit shall be self-contained and consist of a message board, controller, power supply, electric cable, and adjustable height structural support system. The PCMS shall be suitable for either moving on a truck or two-wheeled trailer (3). The MUTCD (4) states that PCMSs mounted on trailers or large trucks should have a minimum letter height of 450 mm (18 in.). CMSs mounted on service patrol trucks should have a minimum height of 250 mm (10 in.). Each character should consist of a matrix at least five pixels wide and seven pixels high. The color of the elements should be yellow or orange on a black background. In addition, research suggests the following guidelines for CMS use: Device format should permit maximum amount of information display at a glance. CMS devices should be located 0.75 mi in advance of closure. CMS devices are to be considered supplemental to currently applied standard traffic control device schemes. CMS devices are not to be considered as an alternative to the arrow panel (5). Design Issues None. Cross References Sign Legibility, 13-10 Key References 1. Dudek, C. (2002). Guidelines for changeable message sign messages. Presentation at theTMC Pooled-Fund Study Annual Meeting, Arlington, VA. Retrieved on July 21, 2006, from http://tmcpfs.ops.fhwa.dot.gov/meetings/mtg_detail.cfm?id=10 2. Ullman, G.L., Ullman, B.R, Dudek, C.L, Williams, A., and Pesti, G. (2005). Advanced Notification Messages and Use of Sequential Portable Changeable Message Signs in Work Zones. College Station, Texas A&M University. 3. ITE (1988). Portable bulb-type changeable message signs for highway work zones: Proposed equipment standard. ITE Journal, 58(4), 17-20. 4. FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 5. Hanscom, F.R. (1982). Effectiveness of Changeable Message Signing at Freeway Construction Site Lane Closures. Transportation Research Record, 844, 35-41.

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-10 SIGN LEGIBILITY Introduction Sign legibility refers to specific design characteristics of work zone signs that contribute to drivers’ ability to perceive and understand the sign’s message. A number of factors determine the legibility of work zone signs including retroreflectivity (sheeting type), color, letter font, and location of sign (roadside or overhead). The legibility index of various sign sheeting can be used to ensure designs that can accommodate all drivers regardless of age and light conditions. Prismatic sheeting ensures greater retroreflectivity of work zone signs and the addition of fluorescent colors improves the sign conspicuity in daytime low-light conditions such as dusk, dawn, or fog conditions. Design Guidelines Studies conducted in Texas (1) have the following findings: Color Overall, yellow and white backgrounds on signs provide the greatest legibility distances followed by green and then orange backgrounds. The MUTCD requires the use of an orange background and black letters in work zone signs (2). Retroreflectivity (Sheeting Type) Fluorescent microprismatic sheeting with orange background provide for greater legibility distance than high-intensity sheeting. Letter Height A maximum legibility index of 40 ft of distance/in. of letter height should be used; a more conservative value is 33 ft/in, which is especially good for older drivers. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data The figure below describes legibility distance for work zone signs. 8 in Legibility Distance at 320 ft (Conservative Value at 264 ft)

13-11 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Discussion Retroreflectivity: A report by the Virginia Transportation Council has specified that for a prismatic lens retroreflective sheeting material, the specification should include values for the material’s orientation and rotation angles, in addition to its entrance and observation angles (3). For high-speed (usually greater than 50 mi/h) highways, anywhere a critical vehicle maneuver is necessary, and in areas of high to medium visual complexity, higher values of sign luminance are required for safety (4). Another report finds that for existing traffic control devices, the beneficial effects of upgrading the type of sheeting used on barrels, barricades, and vertical panels were demonstrated by increased detection and recognition distances. However, the super-engineering grade offered the most cost-effective and balanced solution for upgrading sheeting (5). Legibility: A study has recommended the use of work zone signs with orange background, micro-prismatic materials, which provide far greater legibility distance than high-intensity ones (6). Micro-prismatic fluorescent orange materials were found to perform better than Type 3 (1). Color: Speed variances tended to decrease at the midpoint and the taper with fluorescent signs relative to standard orange signs. The collision reduction in the overall traffic conflicts from what was expected at all treatment sites was about 7% (6). Design Issues Height of letters used depends on site characteristics such as operating speed (1). Cross References None. Key References 1. Chrysler, S.T., Carlson, P.J., and Hawkins, H.G. (2003). Nighttime legibility of traffic signs as a function of font, color, and retroreflective sheeting. Transportation Research Board 82nd Annual Meeting Compendium of Papers [CD-ROM]. 2 FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. 3. Brich, S.C. (2002). A Determination of the Appropriateness of Virginia’s Retroreflective Sign Sheeting Specification for Fluorescent Orange Construction and Maintenance Signs. Final Report (FHWA/VTRC 03-R5). Charlottesville: Virginia Transportation Research Council. 4. Russell, E.R., and Rys, M. (1992). A Review of Kansas Department of Transportation's Reflective Sheeting Policy. Final Report. Manhattan: Kansas State University. 5. Ahmed, S.A. (1991). Evaluation of Retroreflective Sheetings for Use on Traffic Control Devices at Construction Work Zones. Final Report. Stillwater: Oklahoma State University. 6. Hummer, J.E., and Scheffler, C.R. (1999). Driver performance comparison of fluorescent orange to standard orange work zone traffic signs. Transportation Research Record, 1657, 55-62.

HFG CONSTRUCTION AND WORK ZONES Version 2.0 13-12 DETERMINING WORK ZONE SPEED LIMITS Introduction Work zone speed limits refer to the reduced speed limits used in work zones to maintain safe traffic flow. Vehicle speeds in work zones are influenced by the geometrics of the roadway and the location of various work zone features such as lane closure tapers and work activity. Work zone speed limits within 10 mi/h of normal speed limits have more credibility and have been proven to be safer than speed limits that are 15 to 30 mi/h below the normal speed limit. Design Guidelines Speed and crash studies confirm that large speed limit reductions in work zones are undesirable (1). Speed limit reductions to 10 mi/h below the preconstruction speed limit resulted in the smallest increase in speed variance with the work zone—relative to the speed variance upstream of the work zone—of any of the speed limit strategies studied. Additionally, in rural freeway work zones involving work on or near the traveled way, a 10 mi/h reduction in the work zone speed limit minimized the crash rate increase from the preconstruction period to the construction period. Speed Reductions and Speed Limit A study has found that mean vehicle speed reductions were the greatest in work zones where the speed limit was not reduced, or at least not reduced more than 10 mi/h. Speed limits need to be reduced only in the direction of traffic affected by the work zone when a wide median is between the directions of flow. Lane Widths and Number of Lanes Observed or achieved mean vehicle speed reduction appears to be highly correlated with the number of open lanes and lane widths. Speed Display CMSs with radar were successful in causing speeders to slow down in work zones. However, use of work zone speed limit signs with flash beacons produced mixed results. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data Speed considerations in work zones Increase in fatal plus injury crash rates from before construction to during construction. 98.6 4.1 147.9 112.5 0 20 40 60 80 100 120 140 160 0 10 15 20 25 30 Speed Limit Reduction (mph) 0 10 15 20 25 30 Speed Limit Reduction (mph) P er ce n ta g e In cr ea se s (% ) Increase in speed variance from upstream of work zone to work zone. 61.2 34.1 86.7 82.6 92.6 80.6 0 20 40 60 80 100 120 140 160 P er ce nt ag e In cr ea se s (% )

13-13 HFG CONSTRUCTION AND WORK ZONES Version 2.0 Discussion Speed reductions and speed limit: Studies have found that speed limit compliance decreased when the speed limit was reduced by more than 10 mi/h. Mean speeds were approximately 5 mi/h lower within work zones with no speed limit reduction than they were upstream of the same work zones. Speed limit compliance was found to be the greatest in work zones where the speed limit was not reduced (1). Another study noted that the mean and 85th percentile speeds were approximately 9 mi/h lower within the work zones of speed limit reductions of 10 mi/h than upstream of those same work zones and showed that the entire traffic stream uniformly reduced speeds (2). In general, speed reduction is better achieved when the work zone is well marked in advance of the work zone activity; motorists slow down out of self preservation and not the speed limit. Note that people drive the speed they feel comfortable with regardless of the posted speed limit if enforcement is not present (3), whereas speed reduction as high as 9.1 mi/h was observed with the presence of police (4). Lane widths and number of lanes: Lane widths are directly related to speed reduction on roadways. For 11-ft lanes, speed reduction of 4.4 mi/h was observed to be 133% more than the value of 1.9 mi/h recommended by the Highway Capacity Manual (HCM) (5) for basic freeways. For 10.5-ft lanes, the observed reduction of 7.2 mi/h was 69% greater than the value of 4.25 mi/h recommended by the HCM (6). In addition, speed reduction appears to be highly correlated with the number of open lanes. Motorists tend to select higher speeds, regardless of the posted work zone speed limit, when more lanes are open to traffic (7). Speed display: CMSs with radar were successful in effecting significant speed reduction in work zones. Also, no significant differences were found to exist in the speed reductions between vehicle types (8). However, another study found that the use of work zone speed limit signs with flashing beacons produced mixed results. Speed reductions were insignificant on urban arterials where commercial advertisements and other traffic control devices compete for drivers’ attention (9). Design Issues A work zone speed limit may also be affected by restrictive geometric features such as curves or intersections. Cross References Influence of Speed on Sight Distance, 5-12 Key References 1. Migletz, J., Graham, J., Hess, B., Anderson, I., Harwood, D., and Bauer, K. (1998). Effectiveness and Implementability of Procedures for Setting Work Zone Speed Limits. Final Report, NCHRP Project 3-41(2). Independence, MO: Graham–Migletz Enterprises. 2. Sisiopiku, V.P., Lyles, R.W., Krunz, M., Yang, Q., Akin, D., and Abbasi, M. (1999). Study of Speed Patterns in Work Zones. East Lansing: Michigan State University. 3. Fors, C. (2000). Work zone ahead: reduce speed. Roads and Bridges, 38(1), 58-61. 4. Lyles, R.W., and Sisiopiku, V.P. (1999). An Evaluation of Speed Control Techniques in Work Zones (Work Zones 2). East Lansing: Michigan State University. 5. Transportation Research Board (2010). Highway Capacity Manual 2010. Washington, DC. 6. Chitturi, M.V., and Benekohal, R.F. (2005). Effect of lane width on speeds of cars and heavy vehicles in work zones. Transportation Research Record, 1920, 41-48. 7. Sawaya, O.B., and Schofer, J.L. (2000). Speed up, or slow down. World Highways/Routes Du Monde, 9(6), 78-79. 8. Fontaine, M.D., and Garber, N.J. (1996). Controlling Vehicle Speeds in Work Zones: Effectiveness of Changeable Message Signs With Radar (UVA/529242/CE96/102). Charlottesville: University of Virginia. 9. Hall, J., and Wrage, E. (1997). Controlling Vehicle Speeds in Highway Construction Zones (NMSHTD-97-07). Albuquerque: University of New Mexico.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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