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54 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections The most distinguishing design features of a speed table are the flat section in the middle of the table, the ramps on each end, and the aesthetic treatments often applied to the top. The entire speed table is typically 22 ft long in the direction of travel, with a 10-ft flat section in the middle and 6-ft ramps on both ends. The vertical deflection of a speed table typically ranges from three to four inches, but has been designed for a height of six inches in some cases. The ramps on each side of the speed table typically have a parabolic or linear shape. (ITE, 2007) Plateaus should be located approximately 50 to 100 m (165 to 325 ft) from the intersection and designed for a speed of 40 km/h (25 mph). (Schermers and van Vliet, 2001) 4.10.4 Speed Effects Much of the information about the effectiveness of speed tables relates to roadway segments with operating speeds less than 45 mph. No information was found that describes the affect speed tables have on speeds at intersections on high-speed facilities. Most research has shown that speeds were reduced after speed tables were installed. A study in Gwinnett County, Georgia, in which 43 speed tables were installed, found that speeds were reduced by an average of 9 mph. (Bretherton, 2003) Another study reported an average 18% decrease in the 85th-percentile travel speeds for a sample of 58 test sites. (Fehr & Peers, 2007) However, in some cases speed tables may be too gentle to mitigate excessive speed. A test site in Fort Lauderdale, Florida, found no speed reduction. (ITE, 2007) When designed to the typical dimensions, speed tables have an 85th-percentile speed of 25 to 30 mph. (ITE, 1999) Although the speed reduction potential of speed tables is not well known, the ITE (1999) study noted above suggests that the provision of speed tables results in an 85th-percentile speed of 25 to 30 mph. Thus, if a speed table is used at an intersection, the speed reduction on the intersection approach may be from the 85th-percentile mid-block speed to 25 to 30 mph. It is expected that most intersections where speed tables have been used have moderate mid-block or approach speeds. Speed tables located at intersections and mid-block locations have been shown to reduce speeds and collisions as well as to lower traffic volumes. 4.10.5 Safety Effects Speed tables located at intersections and mid-block locations have been shown to reduce speeds and collisions as well as lower traffic volumes. Some studies have reported that collisions have been reduced by an average of 45% on streets treated with speed tables at particular test sites. (ITE, 2007; Fehr & Peers, 2007) Speed tables have potential to increase safety on the treated road by lowering volumes and reduc- ing the probability of a crash. The study conducted in Gwinnett County showed a 7% reduction in traffic volumes based on 24-hour traffic counts collected on 18 streets before and after speed tables were installed. (Bretherton, 2003) ITE reported a 12% reduction in traffic volumes on roadways with speed table applications, depending on the availability of alternative routes. (ITE, 2007) 4.11 Reduced Lane Width 4.11.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Reduced lane widths heighten driver attention by narrowing the avail- able lane width. These treatments can be applied on intersection approaches. Design variations
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Treatment Descriptions 55 Exhibit 4-23. Reduced lane width--painted. can involve re-striping only or narrowing the paved section. Secondary effects and considera- tions should include using with caution when heavy truck traffic or multilane facilities are pres- ent and where curvilinear alignments exist. 4.11.2 Applicability and Considerations Reduced lane width could come in two basic forms: lanes narrowed with paint (see Exhibit 4-23) or a physically reduced roadbed width (see Exhibit 4-24). Lane narrowing benefits using paint may be negated by the relative open field of vision. Narrow roadbeds physically constrain the cross sec- tion but may have secondary impacts. Lane widths that are considered "reduced" tend to range from 9 to 12 ft. Lane-width reductions have been used in both temporary and permanent applications. Per- manent lane width reductions that reduce the overall pavement width are expected to have the greatest potential to reduce speeds because both the pavement width and the striping contribute to drivers' perceptions of the roadway environment. Lane-width reductions have been docu- mented for work zone applications and low-speed urban and residential locations. Lane widths on multilane facilities, roadways with curvilinear alignments, and facilities with high heavy truck or transit vehicle use should be reduced with caution, particularly if the width of the paved section will be reduced. Reducing lane widths can negatively impact bicyclists if bicycle lanes or wide curb lanes are not maintained, but it can also improve bicyclist conditions if the additional space is used to provide or widen bicycle lanes. Reducing lane width can pro- vide positive effects such as space for other roadway features (i.e., medians and curbside park- ing), space for roadside features (i.e., sidewalks and clear zones), and reduced interference with existing roadside development. (Zegeer et al., 2002) Exhibit 4-24. Reduced lane width--roadbed.
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56 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Reduced lane widths improve pedestrian conditions at intersections by reducing crossing distances and exposure time. Excessive crossing distances increase pedestrian exposure time, heighten the potential for vehiclepedestrian conflicts, and may add to vehicle delay. At signal- ized intersection approaches, reducing lane widths and thereby the pedestrian crossing distance provides more flexibility for the intersection's signal timing. 4.11.3 Treatment Layout/Design NCHRP Project 3-72, "Lane Widths, Channelized Right Turns, and Right-Turn Deceleration Lanes in Urban and Suburban Areas," evaluated the operational, speed, and safety effects of reduced lane widths on roadway segments and intersections. The research findings indicate that reducing lane widths to less than 10 ft may not be advisable on four-lane undivided arterials or on the approaches to four-leg stop-controlled intersections. Additionally, it suggests that it may not be advisable to reduce lane widths to less than 9 ft on four-lane divided arterials. These find- ings are consistent with the "Green Book" (AASHTO, 2004), which recommends lane widths of 10 to 12 ft on urban and suburban arterials. Redesigned lanes can be reduced using concrete barriers (for short-term solutions), curbs, or standard striping. The effects of reducing the width of pavement on turning radii at inter- sections should be evaluated to ensure that adequate radii are available to accommodate the design vehicle. 4.11.4 Speed Effects Research related to the speed, safety, and operational effects of reduced lane widths provides inconsistent results, indicating that the relationships are complex and difficult to evaluate with- out considering other elements of the intersection or roadway environment. Research and analysis conducted for NCHRP Project 3-72, which included both high-speed and low-speed facilities, made the following conclusions regarding the speed effects of reduced lane widths: · Reduces mid-block speeds on four-lane arterials (average lane-width reduction of 2.7 ft asso- ciated with average speed reduction of 4 mph), · Reduces driver comfort on higher-speed facilities, · May decrease capacity due to reduced saturation flow rates, although the calculated reduc- tions were about half of the reductions suggested in the Highway Capacity Manual (HCM), and · May increase capacity at signalized intersections due to decreases in pedestrian crossing times. Research that evaluates how effectively reduced lane widths reduce speeds in work zones found that the treatment was most effective on two-lane rural roads and found that a 7% reduc- tion in speed was achieved by reducing lane widths to 11.5 and to 12.5 ft. (Benekohal et al., 1992) In low-speed environments, research has concluded that speed is not consistently related to lane width. (Gattis, 1999) It is also likely that wider lanes induce faster travel speeds that may increase crash risk and increase crash severity. Earlier editions of the HCM suggested that wider lanes on multilane highways also increase capacity and, therefore, reduce following distances. (TRB, 1985) How- ever, HCM editions since 1985 have indicated that wider lanes of up to 12 ft on multilane highways increase free-flow speeds but do not increase capacity and, therefore, do not reduce vehicle headways. (TRB, 1994; TRB, 2000)