Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 20
21 chapter four Cross-Section Elements Overview that platooning decreased and eventually stabilized after a vehicle entered the passing lane. They observed that passing Researchers investigated operational and safety character- activity was greatest at the beginning of the segments and istics of passing lanes on two-lane highways, which grew the greatest benefits of decreased platooning and increased in popularity during the decade in an attempt to maximize safety occurred within the first 0.9 mi of a passing lane seg- performance without widening to traditional four-lane high- ment. Speed patterns were found to vary among sites, but ways. At the same time, the concept of the "road diet" was average speed rose when a vehicle entered the passing lane introduced to reduce traffic speeds and provide space for section. Their study of crash rates included "five years of pedestrian or bicycle amenities without widening the right- crash data from 19 sites; [they concluded that] even though of-way. The effects of the width of travel lanes and TWLTLs the volumes for the passing lane segments were higher than were examined. Shoulder treatments such as rumble strips the state average volume for rural two-lane roads, the pass- received attention for their potential in reducing crashes, and ing lane crash rates were generally lower than the statewide researchers looked at new median and roadside treatments. average crash rate for rural two-lane roads." A study of similar roadways in Texas, called "Super 2" Allocation of Traveled Way Width highways, found that passing lanes were beneficial at volumes approaching 15,000 vehicles per day, particularly on rolling An NCHRP-sponsored scan tour looked at characteristics terrain; the presence of passing lanes improved delay and per- of 2+1 roads in several European countries to determine the cent time spent following (Brewer et al. 2011). Most passing potential applications of the design for use in the United occurred within the first mile of a passing lane, so additional States (Potts and Harwood 2003). A 2+1 road design has a length may be less useful than additional lanes in a Super 2 continuous three-lane cross section with alternating pass- corridor, particularly at lower volumes. Empirical Bayes ing lanes. Although specifics of the designs in the respective analysis of crash data showed that there was a statistically countries varied somewhat, the authors made comparisons significant crash reduction of 35% for segment-only (i.e., of some of the key design and operational criteria, which are nonintersection) injury crashes on the study corridors, as summarized in Table 6. compared with the expected number of crashes without pass- ing lanes. In lieu of guidelines related to specific ADT values, The NCHRP authors concluded that the benefits of 2+1 researchers recommended including general principles for roads in Europe validated a recommendation for their use Super 2 design as part of their proposed revisions to the Texas in the United States, to serve as an intermediate treatment Roadway Design Manual, such as avoiding intersections with between an alignment with periodic passing lanes and a full state highways and high-volume county roads within passing four-lane alignment. They also recommended that 2+1 roads lanes, consideration of terrain and right-of-way in determin- were most suitable for level and rolling terrain, with installa- ing alignment and placement of passing lanes, avoiding the tions to be considered on roadways with traffic flow rates of termination of passing lanes on uphill grades, and discourag- no more than 1,200 veh/hr in a single direction. The authors ing passing lane lengths longer than 4 mi. discouraged the use of cable barrier as a separator, and they recommended that major intersections be located in the buf- Volume 4 of NCHRP Report 500 (Neuman et al. 2003b) fer or transition areas between opposing passing lanes, with discusses the use of center TWLTL on four-lane and two- the center lane used as a turning lane. lane roads to reduce the likelihood of head-on and rear-end collisions. Gattis et al. (2006) reported on a study of passing lane operations in Arkansas. The focus was on segments of con- It can be accomplished either by the conversion of four-lane tinuous three-lane cross sections with alternating passing undivided arterials to three-lane roadways with a center left-turn lanes; for example, three-lane alternate passing or 2+1. They lane or by the more conventional reconstruction of a two-lane road to include the TWLTL. Since the latter could be a costly con- examined the effects of passing lane length on platooning, version because it may require new right-of-way, the four-lane passing, speed, and passing lane crash rates. Five sets of field road conversion is considered more appropriate to the AASHTO data were collected at four rural sites, and it was determined emphasis on low-cost alternatives. However, where right-of-way
OCR for page 21
22 Table 6 Comparison of European 2+1 Road Characteristics Germany Finland Sweden Critical Transition Length, m (ft) 180 500 300 (590) (1,600) (1,000) Non-Critical Transition Length, m (ft) 3050 50 100 (100160) (160) (330) Typical Passing Lane Length, km (mi) 1.01.4 1.5 1.02.0 (0.60.9) (0.9) (0.61.2) Separation between Opposing Traffic 0.5 0.3 1.252.0 m (ft) (1.6) (1.0) (4.16.6) Fatal+Injury Crash Rate, per 106 veh- 0.16 0.09 0.50 km (106 veh-mi) (0.26) (0.14) (0.80) Typical Volumes, veh/day 15,000 14,000 4,000 25,000 25,000 20,000 Source: Potts and Harwood (2003). Note: Sweden's 2+1 roads are separated by cable barrier, and their crash rates are specifically reported as crashes per million axle pair-km. cost is not a major consideration, the inclusion of TWLTLs on According to Kuhn et al. (2005), "in most cases, the existing two-lane roads may be an even more effective treatment design speed of managed lanes will be the same as that used for head-on collisions since more of such collisions would likely occur on two-lane roads than on four-lane roads. on the adjacent general-purpose lanes. However, there may The development of TWLTLs is usually for traffic operations be limited instances where the design speed of the man- rather than safety concerns. TWLTLs are usually implemented to aged lanes is lower than the adjacent general-purpose lanes, improve access. When they are used in response to a safety con- owing to the geometrics of the managed lanes facility or cern, the use is traditionally to reduce driveway-related turning and rear-end collisions. However, because studies have also indi- other limitations. The designated design speed of the facility cated a positive effect on head-on crashes, the strategy is included should relate to the maximum speed the facility is expected here. The principle behind the use of TWLTLs in this context is to to accommodate. Further, the design speed should accom- provide a buffer between opposing directions of travel. The strat- egy is intended to reduce head-on crashes by keeping vehicles modate the vast majority of users (e.g., the anticipated 85th from encroaching into opposing traffic lanes through the use of percentile speed)." the buffer. The Managed Lanes Handbook also contains recommen- If available right-of-way, construction budget, and traffic vol- dations on horizontal and vertical clearance and curvature, umes allow, incorporating TWLTLs into the design of new gradient, SSD, cross slope, superelevation, and minimum and reconstructed roads is more efficient than converting road- turning radius. Their recommended guidelines are summa- ways later. rized in Table 7. Managed Lanes Hard Shoulder Running Transportation agencies in many jurisdictions have exam- In the United States, the primary use of shoulders has been ined new ways to maximize the use of existing infrastruc- as a safety refuge area; however, in recent years there has ture to improve capacity and reduce congestion, particularly been an increasing trend for transportation agencies to on urban freeway corridors. One such method is the use of explore the use of shoulders as travel lanes during peak managed lanes, whereby one or more lanes in the corridor periods as a congestion management strategy. Also called are reserved for use that is limited to specific types of vehi- "hard shoulder running," the limited use of the shoulder as cles, such as high-occupancy vehicles (HOV), buses, motor- a travel lane has been primarily reserved for special users cycles, or toll-paying drivers. Kuhn et al. (2005) conducted of the roadway system, most often transit vehicles. Overall, a multi-year study on a wide variety of planning, design, and experience using shoulders for interim use has been posi- operational issues related to managed lanes. A chapter in their tive in the United States, and more agencies are considering Managed Lanes Handbook contains recommendations and the strategy to address growing congestion on their urban guidelines for design elements of freeway managed lanes. freeway networks. Several states have deployed temporary In general, they recommended that the features of the man- shoulder use for all vehicles on congested corridors with aged lane be commensurate with the design vehicle that is success. Kuhn (2010) describes several uses of hard shoul- selected to be appropriate for the facility. They recommended der running in the United States, as noted in the following that the designer use the AASHTO Green Book templates in paragraphs. determining turning paths, lateral and vertical clearances, bus stops, and other elements associated with a project. In particu- In San Diego, California, along I-805/SR 52, transit vehi- lar, the design process might also account for the path of the cles may use the freeway shoulder during congested periods, vehicle overhang beyond the outside turning radius. when general-purpose lane traffic slows to 30 mph or lower.
OCR for page 22
23 Table 7 Summary of Managed Lanes Mainline Design Criteria U.S. Customary Metric Design Speed Desirable Reduced Desirable Reduced (70 mph) (50 mph) (110 km/h) (80 km/h) Alignment Stopping distance 730 ft 425 ft 220 m 130 m Horizontal curvature (radius) 2,0502,345 ft 835930 ft 560635 m 250280 m Maximum superelevation 0.04 ft/ft 0.06 ft/ft 0.04 m/m 0.06 m/m Rate of vertical curvature Crest, k 247 84 74 26 Sag, k 181 96 55 30 Gradients Maximum (%) 4.0 5.0 4.0 5.0 Minimum (%) 0.5 0.3 0.5 0.3 Clearance Vertical 16.5 ft 14.5 ft 5m 4.4 m Lateral 4 ft 2 ft 1.2 m 0.6 m Lane Width Travel lanes 12 ft 11 ft 3.6 m 3.4 m Cross Slope Maximum 0.020 ft/ft 0.020 ft/ft 0.020 m/m 0.020 m/m Minimum 0.015 ft/ft 0.015 ft/ft 0.015 m/m 0.015 m/m Superelevation: Dependent on curve radii and design speed [0.10 ft/ft (0.10 m/m) maximum]. Source: Kuhn et al. (2005). They may travel no more than 10 mph faster than traffic in shoulder, and buses must reenter the main lanes when the general traffic lanes. The cross section of the shoulder is at shoulder is obstructed. Typically, buses may use the shoul- least 10-ft wide throughout the deployment area. Pavement der any time that traffic in the adjacent mainlines is moving markings to indicate the operational strategy include text indi- at less than 35 mph. Buses may travel no more than 15 mph cating "Transit Lane Authorized Buses Only" (Martin 2006). faster than mainline traffic with a 35 mph maximum allowed speed on the shoulder. The typical minimum shoulder width The Florida DOT, MiamiDade Transit, and the Miami is 10 ft, with an 11.5-ft minimum at bridges, and a 12-ft mini- Dade Expressway Authority operate several shoulder use mum on new construction. Typically, buses travel through applications in the Miami region. Along the Florida Turn- the entrance and exit ramps; where queues are long at ramps pike, SR 826, and SR 836, buses are allowed to use the with metering, buses typically merge with traffic on the ramp shoulder when the freeway is congested. Implemented in and return to the shoulder after the ramp (Kuhn 2010). 2005, the program stipulates that buses may travel no faster than 35 mph on the shoulder when open to transit. The typi- One segment of I-35W in Minneapolis has a unique combi- cal cross section is a minimum 10-ft width, with a 12-ft nation of strategies. Known as priced dynamic shoulder lanes width in high volume areas, and cross slopes of 2% to 6%. (PDSL), the left shoulder is open during the peak periods; Pavement markings indicate "Watch for Buses on Shoulder" transit and carpools use the shoulder for free and MnPASS (Martin 2006). customers can use the shoulder for a fee. As shown in Figure 4, the left shoulder is open to traffic, with overhead sign gan- On GA 400 in Alpharetta, Georgia, buses are allowed tries indicating its operational status. When the general- to use the freeway shoulder in an effort to provide access purpose lanes become congested, the shoulder is opened between a local transit rail station and a park-and-ride lot. and the speed limit on the general-purpose lanes is reduced The Georgia Regional Transportation Authority and Georgia (Kuhn 2010). DOT operate the facility, which is functional whenever traf- fic slows to 35 mph or less. Buses can travel no more than In dedicated shoulder-lane operations, either general- 15 mph faster than general-purpose lane traffic and are required purpose or HOV-specific capacity has been added through to reenter general traffic lanes before interchanges. A con- the permanent conversion of shoulders. Most HOV appli- struction project was necessary to upgrade the travel surface by cations use the interior lane for HOV operations, whereas widening the shoulder by 2 ft and providing reinforcement of the exterior shoulder is used for general-purpose traffic so the shoulder pavement (Martin 2006). as to maintain the same number of general-purpose lanes that existed before implementation. A typical HOV appli- An extensive system around MinneapolisSt. Paul, Min- cation would convert a three-lane freeway with 12-ft lanes, nesota, is also focused on buses. The operational strategy 10-ft exterior shoulder, and 8-ft interior shoulder to 11-ft is considered interim, and some deployments have been general-purpose lanes, 14-ft (including buffer striping) removed since their inception. When operational, buses must HOV lane, 5-ft exterior shoulder, and 2-ft interior shoulder yield to any vehicle entering, merging, or exiting through the (Kuhn 2010).