Work Zone Speed Management (2015)

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49 chapter five OPERATIONAL SPEED MANAGEMENT TECHNIQUES INTRODUCTION Operational speed management techniques utilize strategi- cally placed speed control vehicles (operated by or on behalf of the highway agency) to limit work zone traffic speeds. Three fairly distinct operational techniques were identified: pilot vehicles, pace vehicles, and rolling closures. A brief dis- cussion of the use of flag persons to limit vehicle speeds is also included in this chapter. PILOT VEHICLES Pilot car or pilot vehicle operations (also called convoy work- ing in some countries) involve stopping traffic in advance of a work zone and then using an appropriately signed work vehicle to lead traffic through the site. The use of pilot vehicles is mentioned in the 2009 MUTCD as a method for guiding a queue of vehicles through a temporary traffic control zone or detour. In the United States, the method is typically applied to special situations such as two-way, one-lane operations in mountainous areas. For example, PennDOT reports the use of pilot vehicles for work zones on two-lane highways, especially in areas where it is difficult for flaggers to communicate with each other because of the length of the work zone or because mountainous terrain inhibits radio communication (M. Briggs, PennDOT, personal communication, 2014). Similarly, VDOT reports the use of pilot vehicles for two-way, one-lane opera- tions that are more than two miles long or where flaggers can- not see each other because of sharp curves (D. Rush, VDOT, personal communication, 2014). Pilot vehicles also provide an affirmative method for limiting work zone speeds, because vehicles in the convoy are normally prohibited from overtaking (passing) the pilot vehicle. VDOT uses pilot vehicles for this purpose when it is necessary to maintain low-speed traffic on poor running surfaces. Some countries appear to make greater use of pilot vehi- cles than the United States. For example, the MUTCD for Queensland state in Australia identifies four situations where a pilot vehicle may be used: 1. Part of the length of the work site is out of view of the supervisor, work crew, and the flagger. 2. The hazard to workers described requires the traffic speed to be reduced to less than 40 km/h (25 mph). 3. The traffic speed is required to be kept low to minimize damage to the work. 4. Traffic is to follow a particular path through the site, which may not be obvious unless a pilot vehicle is used. In the United Kingdom, the Traffic Signs Manual (DfT 2013a) provides detailed advice on the use of pilot vehicles for situations where “normal traffic management arrange- ments are not feasible because of restricted highway width, and diversion is impracticable.” The manual allows speeds to be reduced to 10 mph or less when the convoy is passing an especially hazardous location such as a work area with “little or no safety zone clearance.” The manual also suggests using pilot vehicles during preventative maintenance opera- tions “when it is considered necessary to ensure compliance with speed limits which have been implemented to protect newly laid surface dressing.” The British guidance does not allow pilot vehicles to be used on freeways; however, they are allowed day and night on other divided and undivided highways when traffic is reduced to a single lane in either or both directions. Relatively little academic research has been conducted on the use of pilot vehicles; however, a recent study evalu- ated their effectiveness for speed reduction at a long-term, rural highway work zone in Queensland, Australia (Debnath et al. 2014). In contrast to typical U.S. practice, the 2.5-mile- study segment was straight and mostly flat with good sight distance. The ordinary speed limit was 80 to 100 km/h (50 to 62 mph). Analysis of data covering a 5-day period showed that a pilot vehicle reduced the average speeds at the treat- ment location. There was no “halo effect” downstream of the work zone. The portion of vehicles speeding through the activity area was also reduced, particularly those trav- eling at 10 km/h (6 mph) or more above the posted limit. Drivers were less likely to speed when the proportion of large (medium and heavy) vehicles in the traffic stream was greater. Medium vehicles (two- and three-axle buses or trucks, four-axle trucks) were less likely to speed in the presence of a pilot car than light vehicles. When a pilot car was present, the mean speed of all vehicles under a posted speed limit of 40 km/h dropped from 52.0 to 46.1 km/h (32.3 to 28.6 mph), a reduction of 5.9 km/h (3.7 mph). Despite these reductions, the mean speed of all vehicles remained 6.1 km/h (3.8 mph) above the posted limit when the pilot car was in operation, primarily because in many instances the pilot vehicle was itself speeding. To realize the full benefit of pilot vehicle operations, the pilot vehicles must observe the speed limit.

50 as freeway paving operations, where temporary concrete bar- riers cannot be provided, and yet workers must work within 5 to 10 ft of a live lane of traffic. The Ontario Traffic Man- ual devotes several pages to the topic of pace vehicles and stipulates that either police or pace vehicles must be used to support an 80 km/h (50 mph) speed limit during freeway paving operations, unless the traffic volume is sufficient to keep speeds below that threshold (Ministry of Transportation– Ontario 2014). For continuous pacing of the work zone (e.g., throughout a work shift), the pace vehicles are dispatched at a predetermined rate in pairs (for a two-lane work zone) or trios (for a three- lane work zone) (Figure 32). Typically, the vehicles enter the roadway at an upstream interchange. As they approach the work zone the pace vehicles begin traveling side-by-side to prevent other traffic from overtaking (passing) them. The pace vehicles gradually reduce speed as they approach the work zone and proceed through the work zone at a predetermined speed (e.g., the work zone speed limit). Downstream of the work zone the vehicles gradually accelerate to the ordinary speed limit and prepare to exit the roadway. If the roadway is being paced in both directions, the vehicles then repeat the operation for the opposite direction; if only one direction is being paced, the vehicles return to their starting point and prepare to lead another platoon of traffic through the work zone. Thus, the total number of vehicles required is a function of the number of lanes that are open to traffic, the round-trip travel time, and the desired headway (time interval) between pace vehicles. In some cases, a dispatcher or monitor may be used to ensure that the headway remains reasonably uniform, and a reserve driver may assist the primary drivers during their meals and breaks. Although there appears to be little doubt about the poten- tial for pace vehicles to limit traffic speeds to very low val- ues (or bring the traffic to a complete stop if necessary), little formal research has been conducted on their use and little A survey and interview of Canadian provincial transpor- tation ministries requested information about the perceived effectiveness of pace vehicles and pilot vehicles (Harmelink and Edwards 2005). As shown in Table 10, 90% of the agen- cies rated the effectiveness as “high” or “medium.” Only police enforcement received similarly high scores. Harmelink and Edwards concluded that pilot vehicles are “very effec- tive methods of controlling motorist speeds in work zones, without requiring police enforcement” when used together with flaggers on a two-lane road to guide a queue of vehicles through a one-lane section of a temporary traffic control zone or detour, or to control the speed of vehicles through the con- struction site, especially immediately adjacent to areas where workers are present. Oregon DOT reports stipulating the maximum speeds of vehicles that are making materials deliveries to work zones on rural, low-volume facilities (Pappe 2014). The effective- ness of this less formal type of speed-controlling operation probably depends on the frequency and uniformity of the deliveries (and the extent to which delivery drivers comply with the stipulations). Such methods can potentially be used to augment more formal speed pacing. PACE VEHICLES Pace vehicles are conceptually similar to pilot vehicles; how- ever, the term pace vehicles typically refers to speed con- trol for freeways and other multilane divided highways. The phrase appears to have been borrowed from automobile rac- ing, where pace cars have long been used to limit speeds during adverse track conditions. In the work zone context, Harmelink and Edwards (2005) provide the following definition: One or more pace vehicles are used on freeways, one pace vehi- cle per open lane of traffic, to constrain and control the speed of vehicles traveling through the work zone, where reduced speed is necessary but it is difficult to achieve speed reductions by other means. Currently, the use of pace vehicles in freeway situations is uncommon in the United States. A 2005 Canadian study advocated their use (Harmelink and Edwards 2005). The authors of that report suggest the use of pace vehicles (subject to approval by the road authority) to control speeds in long- duration freeway construction work zones in situations, such Rating Number of Agencies Percent of Agencies High 14 67 Medium 5 24 Low 2 10 Not Effective 0 0 Source: Harmelink and Edwards (2005). TABLE 10 PERCEIVED EFFECTIVENESS OF PACE VEHICLES AND PILOT VEHICLES AMONG CANADIAN PROVINCIAL MINISTRIES OF TRANSPORTATION FIGURE 32 Although not typically intended as a speed control measure, snow removal operations often result in situations where traffic is “paced” by maintenance vehicles (Viking-Cives 2014).

51 work site, allow the convoy to resume normal speed gradu- ally, and exit the roadway downstream. Advance signage and other details of this operation are critical to its safety, and have been documented in some detail by Harmelink, America Traf- fic Safety Services Association (ATSSA), and in the British Traffic Signs Manual (DfT 2013b), among others. The specific type of pace vehicles used for rolling closures varies by state. In some jurisdictions the vehicles are operated only by police, whereas in others maintenance or contractor vehicles are also considered acceptable, provided that they are equipped with appropriate signage and flashing lights or beacons or arrow boards. Harmelink and Edwards (2005) and ATSSA (2013) both assert that driver compliance is likely to be higher when the closure is paced by a police vehicle than when paced by a contractor vehicle. Some jurisdictions use a combination of police and contractor vehicles; for example, a March 2012 policy drawing by the Kentucky Transportation Cabinet shows the use of at least four vehicles, of which two must be police (KYTC 2012). VDOT uses one police vehicle and two to three contractor vehicles depending on the num- ber of lanes to be closed (D. Rush, VDOT, personal commu- nication, 2014). The Ontario Traffic Manual stipulates that the back of each pace vehicle must be equipped with “do not pass” signage (Ministry of Transportation–Ontario 2014). Complete closure of the entrance ramps in the rolling clo- sure area is particularly important to the safety of the opera- tion, since any vehicle that enters the freeway downstream of the paced platoon has the potential to arrive at the work site at full freeway speed. Some agencies assert that a police presence is helpful in ensuring that drivers respect the ramp closures; for example, South Carolina DOT requires the ramp closure to be done by uniformed law enforcement per- sonnel in police vehicles with lights flashing (J. Sease, per- sonal communication, 2014). Conversely, Kentucky allows contractor vehicles to be used for this purpose (KYTC 2012). Little formal research appears to have been conducted on the desirable minimum speed the pace vehicles would use in rolling closures. Kentucky Transportation Cabinet calls for a minimum speed of 10 mph (KYTC 2012). ATSSA suggests a minimum speed of 10 mph, with preferred speeds of 20 to 30 mph (ATSSA 2013). The Traffic Signs Manual for the United Kingdom requires a pace vehicle speed in the 20 to 30 mph range (DfT 2013a). The Ontario Traffic Manual takes a more conservative approach, stipulating that on rural low- volume freeways the pace vehicle should lead at a speed of no less than 15 to 20 km/h (9–12 mph) below the normal posted regulatory speed (Ministry of Transportation–Ontario 2014); this would correspond to a pacing speed of 50 to 55 mph on a freeway that normally operates at 65 mph (the Ontario Traf- fic Manual is less prescriptive in the case of urban freeways, on the premise that urban drivers are more accustomed to congestion). Presumably, the maximum safe reduction in the running speed also depends on the driving environment and the type of signage that is provided in advance of the closure, guidance has been developed for multilane applications other than rolling closures (which are discussed in the next section). Additional research may be desirable to: • Identify a more complete set of work zone situations that are amenable to the use of pace vehicles. • Determine the minimum practical speed for the pacing operation and its relationship to traffic volume. • Determine the optimal headway for the pace vehicles in relation to the length of the work zone. • Determine the vehicle markings that are most desirable for the construction and maintenance vehicles used for this application. • Determine whether controlled acceleration and decel- eration by the pace vehicles can delay the onset of work zone congestion. ROLLING CLOSURES A special case of the use of pace vehicles is the rolling clo- sure, also called a rolling roadblock, rolling slowdown, slow roll, or mobile carriageway closure. This procedure is used to create a time window when the road downstream of the lead vehicles is effectively clear of vehicles (traffic volumes must be low to avoid creating an excessively long bottleneck). A number of DOTs use this procedure for very short-duration work operations on freeways and other multilane divided high- ways. Examples of such work include: • Installing or removing a sign bridge, • Setting a bridge girder, • Stringing electrical lines across the roadway, • Switching a freeway lane closure from a right-side clo- sure to a left-side closure, • Changing lane shifts in multiphased projects, and • Removing debris from the roadway. As illustrated in Figure 33, in a rolling closure a group of designated pace vehicles (at least one per lane and possibly including one for each drivable shoulder) enters the traffic stream. After synchronizing their positions across all lanes to eliminate all opportunities for being overtaken by other traf- fic, the pace vehicles gradually reduce their speeds to create an interval of approximately 5 to 10 minutes where the travelled way will be clear of all traffic upstream of the work site. The speed differential between downstream traffic moving away from the work site and the reduced-speed convoy approach- ing the site creates the working window. Consequently, work cannot commence until a monitor has verified that all down- stream traffic has cleared the work site. In addition, all per- sonnel and equipment must clear the travelled way before the convoy can pass through the site. In the unfavorable case that the work is incomplete when the pace vehicles reach the work site, the pace vehicles bring the convoy to a complete stop upstream of the site. In the normal case where the work has been completed on time, the pace vehicles continue past the

52 FIGURE 33 Rolling roadblock diagram from British Traffic Signs Manual (DfT 2013b).

53 work zone speeds is generally considered to be a supplemen- tal (rather than a primary) method of encouraging drivers to slow down. The technique is perhaps most applicable to situ- ations where a flagger is already present on the site (e.g., as a spotter). Concerns over worker safety and labor cost appear to be diminishing the use of this technique in the United States, especially on high-speed roadways and at night. Mid-1980s research suggested that a properly trained flag- ger simultaneously displaying the MUTCD slow sign, mak- ing the slow-down hand gesture, and positioned adjacent to a speed limit sign, could reduce average speeds (Richards et al. 1984; Bryden and Mace 2002). The technique also included having the flagger point at a reduced speed limit sign with his or her free hand. Work zone speeds were reported to have been reduced by 4 to 5 mph on urban interstates, 7 to 13 mph on rural interstates, 13 mph on urban arterials, and 10 to 16 mph on rural, two-lane highways. An early 1990s study evaluated the effect of flaggers on traffic speed along a rural Illinois work zone (Benekohal and Kastei 1991). The effects of flagger training on their effec- tiveness were also studied by collecting data before and after the flaggers received training. Training included recommen- dations in the MUTCD on proper posturing, motions, asser- tiveness of flaggers, and making eye contact with motorists. The average speeds of cars and trucks were reduced by 11.7 and 9.1 mph before a flagger received training and were reduced by 14.9 and 11.9 mph, respectively, after they received training. A mid-1990s study evaluated traditional and new flagging procedures (McCoy and Bonneson 1993). The new flagging procedure included larger flagger signs and new yellow–green flagger apparel. New and traditional approaches reduced mean speeds by 9.2 and 11.1 mph, respectively. The 2009 MUTCD allows for the use of remotely operated Automated Flagger Assistance Devices (AFADs) for short- term and intermediate-term work activities at sites where there is only one lane of approaching traffic (FHWA 2009). In addition to the safety benefit of removing the flagger from exposure to traffic, the devices offer the potential for a one- person operation in some situations (Pigman et al. 2006). Studies have evaluated several proprietary AFAD products to determine operational effectiveness and quantify failure- to-stop violations, but relatively little research has been con- ducted to identify any differences in the deceleration speed profiles for vehicles approaching AFADs as compared with conventional flagging operations. but neither topic appears to have been researched in detail. Further work to clarify this parameter may be desirable, because the rolling closure may need to start 10 to 15 miles upstream of the work site under the more conservative rural Ontario speed reduction assumptions. Interviews with state DOT officials conducted for this project indicate that in many states the availability of police resources is a constraint on implementation of work zone speed management techniques. Therefore, further research may also be desirable to determine whether specially marked (highly conspicuous) pace vehicles operated by contractors or main- tenance personnel could achieve compliance levels similar to those that occur when police vehicles lead the pace. SPEED LIMITERS It is important that pilot, pace, and rolling closure lead vehicles conform to the intended speed profile to ensure the effective- ness of the operation. Vehicles used in operational speed man- agement applications can potentially benefit from the use of road speed limiters, which allow drivers to adhere to a speci- fied maximum speed without having to constantly monitor their speed. The devices are widely used to limit the top speed of heavy trucks and other fleet vehicles. Various vendors offer fixed and adjustable limiters, both as original equipment and as retrofit kits for existing vehicles (VDO 2008; AutoKontrol 2011; InterMotive 2014; SCT 2014). Speed limiters are also increasingly being offered as a factory-installed option for cars and light-duty trucks, particularly in countries where the use of automated speed enforcement is widespread (Lingeman 2012). In contrast to cruise control (which maintains a constant speed), a speed limiter prevents the driver from accelerating above a set speed; if the driver releases the accelerator the vehicle will slow down (Ford Motor Company 2010). Some speed limiter systems are advisory, activating a warning light or chime if the vehicle exceeds the desired speed; other systems restrict fuel flow if the vehicle begins to exceed the desired speed. FLAGGING FOR SPEED REDUCTION Flaggers (also called flag persons or traffic control persons) are often utilized for two-way, one-lane operations on undi- vided rural highways and urban streets. A second type of flag- ging involves slowing freeway traffic. For example, Figure 46 (in chapter eight) illustrates intermittent flagging to assist con- struction vehicles with re-entering the general traffic stream as they leave a freeway work activity area. Flagging to reduce