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19 evaluated without respect to the sign of the grade. The sign of basis for agency work zone sight distance design criteria is the the grade is irrelevant because each grade on a rural two-lane, set of SSD values from the Green Book. For this case, mini- two-way highway is an upgrade for one direction of travel and mum crest and sag vertical curve lengths are determined from a downgrade for the other. The grade factors are applied to the Exhibits 3-71 and 3-74, respectively, in the 2004 Green Book. entire grade from one point of vertical intersection to the next. The CMFs apply to total roadway segment crashes. Summary of Key Findings Work Zone Considerations This section summarizes key findings from the research noted in this chapter. This is an annotated summary; conclu- NCHRP Report 581 (Mahoney et al. 2004) discusses sev- sions and recommendations are those of the authors of the eral elements of design and their relation to work zones. The references cited. authors state that although extended sight distances through- out work zones are desirable, the underlying need for decision sight distance (because of an unexpected or difficult-to- Stopping Sight Distance perceive information source or condition) should be avoided in designing construction work zones. Temporary traffic control New values for SSD and new design controls for verti- and other driver information strategies are used in conjunction cal curves were recommended, based on a PRT of 2.5 s, with extended sight distance to mitigate work zone condi- a 10th percentile deceleration rate of 11.2 ft/s2, a 10th tions that are atypical or involve complex driver decisions. percentile driver eye height of 3.5 ft and a 10th percen- They concluded "that extended sight distance approach- tile object height of 2.0 ft (Fambro et al. 2000). ing and within work zones is desirable from an operations Ramp control signals placed on the left side of a curve perspective. Safety issues also point to [the need for] some of a loop on-ramp (even with a radius greater than 300 ft) minimum sight distance." For work zone design speeds are more critical for accommodating SSD than those on less than 40 mph, the SSD values tabulated in the Green the right side (Wang 2007). Book and corresponding to work zone design speed were The method of selecting SSD values deterministically recommended. For work zone design speeds of 40 mph and yielded conservative estimates of available and required greater, the Green Book design-speed-corresponding values SSD, resulting in a very low probability (0.302%) of did not necessarily represent the minimum values that could hazard (Sarhan and Hassan 2008). be accepted; a minimum sight distance of 300 ft was rec- ommended using a driver eye height of 3.5 ft and an object Passing Sight Distance height of 2.0 ft. An analysis of observed passing maneuvers provided Maximum superelevation rates (emax) are typically support for the AASHTO PSD model, and the model selected as a matter of policy rather than for specific proj- provided reasonable results for the assumptions made. ects. Absent other considerations, the emax used for perma- However, the model's assumptions may need to be nent roadways is appropriate for construction work zones. updated or accommodate more flexibility for speeds Superelevating roadway curves necessitates superelevation higher than 55 mph (Carlson et al. 2005). transitions, which bring alignment and other (e.g., drainage) Increased consistency between AASHTO PSD design complications. For these reasons, it is common design prac- standards and MUTCD pavement marking practices was tice to provide curves that are sufficiently flat to not require recommended, specifically accomplished by using the the introduction of superelevation. Mahoney et al. (2004) MUTCD criteria for marking passing/no-passing zones discuss the use of Methods 2 and 5 from the Green Book for on two-lane roads in the Green Book's PSD design pro- determining appropriate superelevation distributions. cess. In addition to providing the desired consistency between PSD design and marking practices, two-lane NCHRP Report 581 (Mahoney et al. 2004) also states that, highways could be designed to operate safely with the in general, the same maximum grade criteria applicable to MUTCD criteria (Harwood et al. 2008). the highway under construction should be applied to work zone roads. However, marginally exceeding these criteria is often justified in consideration of all factors. Grades below Horizontal Alignment the maximum are desirable. When designing work zone tem- porary roadways, the potential effect of grades on operations Erroneous perceptions by drivers approaching horizon- and capacity should be considered. When speeds are sub- tal curves, as influenced by vertical curves, increased stantially reduced in advance of a temporary roadway (e.g., as (1) the sight distance increased, (2) the horizontal in conjunction with a reduction in the number of lanes), the curve radius increased, and (3) the length of vertical work zone capacity may be controlled by heavy vehicles curve per 1% change in grade decreased. Drivers tend attempting to accelerate on grade, which, in turn, influences to drive faster on horizontal curves in sag combinations queue formation. The authors found that the most common and slower on horizontal curves in crest combinations.

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20 Designers can establish the profile and predicted oper- Drivers made more errors on horizontal curves that ating speed of an alignment based on a 3-D model, were adjacent to vertical curves, particularly crests rather than a traditional 2-D model (Bidulka et al. 2002; that obscured a downstream horizontal curve. Hassan et al. 2002). There were increased errors when curves were com- For drivers on curves with radii greater than or equal to bined with other elements, especially intersections. 350 m (1,146 ft), as the deflection angle increased, speed measures (mean, 85th percentile, and 95th percentile) Vertical Alignment decreased; as a result, motorists may view a large change in direction as a motivation to slow their speed. In addi- Current North American design practices might yield tion, as curve length increased, speed measures increased, segments of the vertical curve where the driver's view suggesting that drivers may become more comfortable is constrained to a distance shorter than the required at higher speeds because they have more time to adjust SSD. An alternative design procedure is recommended their vehicle path to a constant radius. Grade has an influ- based on a new model that incorporated longitudinal ence on the upper-percentage range of vehicle speeds, friction and acceleration, which produced new recom- because the 85th percentile speed decreased as approach mended values for minimum lengths of crest and sag grade increased (Schurr et al. 2002). vertical curves (Hassan 2004). A study of driver behavior and errors on a selection of A weight/power ratio of 102 to 108 kg/kW (170 to 180 horizontal curves led Lyles and Taylor (2006) to con- lb/hp) would be appropriate for freeways in California clude the following: and Colorado, and a weight/power ratio of 126 kg/kW Where applicable, drivers approaching curves rou- (210 lb/hp) would be more appropriate in Pennsylvania, tinely exceeded the posted speed limit as well as the as compared with the 120 kg/kW (200 lb/hp) value rec- posted advisory speed. ommended in the 2001 Green Book (Torbic et al. 2005). Drivers had more errors at curves where they had The upward divergent headlamp angle used in the sag limited or no visibility of the curves when the TCDs curve design equation should be reduced from 1 to were first visible. between 0.75 and 0.90 (Hawkins and Gogula 2008).