Recent Roadway Geometric Design Research for Improved Safety and Operations (2012)

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66 Background From 2000 through early 2011 a significant amount of geo- metric design-related research was conducted on a wide variety of topics and issues. The objective of this study was to identify and summarize a sample of roadway geometric design literature completed and published during that time, particularly research that identified safety, operations, and maintenance impacts. A national literature review repre- sented the vast majority of the effort for this synthesis study. Summary of findingS The body of this report has five primary chapters, in addition to the introductory chapter and this concluding chapter. This section of the report will present a summary of the key find- ings in the body of the report, categorized by topic. This is an annotated summary of the findings from the research dis- cussed in the body of the report; the recommendations listed are those of the authors of the references cited. It is important to note that the recommendations included in this list of findings from the literature are those of the authors cited. Before any revisions to AASHTO’s Green Book were to be made on the basis of these recommendations, they would need to be considered on the basis of the rigor of the research and logic that underlie them. No endorsement of these recommendations is implied by their inclusion in the listing of findings from the literature. design Vehicles • Dimensions of commonly used trucks have changed in recent years, prompting recommendations to revise the dimensions of those vehicles in AASHTO’s A Policy on Geometric Design of Highways and Streets, com- monly known as the Green Book (Harwood et al. 2003). • Along with the changes in dimensions have come changes in performance; however, design guidelines are sufficient to accommodate their performance for many design elements (Harwood et al. 2003a). design Speed • Posted speed limit and anticipated operating speed were frequently associated with the selection of design speed (Fitzpatrick and Carlson 2002). • Observation of driving behavior revealed that the strong- est indicator of operating speed was posted speed limit. Design speed appeared to have minimal impact on operating speeds unless a tight horizontal radius or a low K-value was present (Fitzpatrick et al. 2003a). • Researchers investigated the possibility of selecting a design speed based more heavily on the context of the environment in which the roadway was located. A primary area of concern, however, was how to define the context to be considered (Garrick and Wang 2005; Wang et al. 2006). driver characteristics • Designers and traffic engineers must examine the road- way environment for information conflicts that may mislead or confuse road users (Campbell et al. 2008). • Designers and traffic engineers must also seek road envi- ronments that are self-explaining, quickly understood, and easy for users to act on (Campbell et al. 2008). Stopping Sight distance • New values for stopping sight distance (SSD) and new design controls for vertical curves were recommended based on a perception–reaction time of 2.5 s, a 10th per- centile deceleration rate of 11.2 ft/s2, a 10th percentile driver eye height of 3.5 ft, and a 10th percentile object height of 2.0 ft (Fambro et al. 2000). • Ramp control signals placed on the left side of a curve of a loop on-ramp (even with a radius greater than 300 ft) are more critical for accommodating SSD than those on the right side (Wang 2007). • The method of selecting SSD values deterministically yielded very conservative estimates of available and required SSD, resulting in a very low probability (0.302%) of hazard (Sarhan and Hassan 2008). Passing Sight distance • An analysis of observed passing maneuvers provided support for the AASHTO passing sight distance (PSD) model, and the model provided reasonable results for the assumptions made. However, the model’s assumptions may need to be updated or accommodate more flexibility for speeds higher than 55 mph (Carlson et al. 2005). chapter seven concluSionS

67 • Increased consistency between AASHTO PSD design standards and Manual on Uniform Traffic Control Devices (MUTCD) pavement marking practices was recommended, specifically accomplished by using the MUTCD criteria for marking passing/no-passing zones on two-lane roads in the Green Book’s PSD design pro- cess. In addition to providing the desired consistency between PSD design and marking practices, two-lane highways could be designed to operate safely with the MUTCD criteria (Harwood et al. 2008). Horizontal alignment • Erroneous perceptions by drivers approaching horizon- tal curves, as influenced by vertical curves, increased as (1) the sight distance increased, (2) the horizontal curve radius increased, and (3) the length of vertical curve per 1% change in grade decreased. Drivers tend to drive faster on horizontal curves in sag combinations and slower on horizontal curves in crest combinations. Designers should establish the profile and predicted operating speed of an alignment based on a three-dimensional model, rather than a traditional two-dimensional model (Bidulka et al. 2002; Hassan et al. 2002). • For drivers on curves with radii greater than or equal to 350 m (1,146 ft), as the deflection angle increased, speed measures (mean, 85th percentile, and 95th percentile) decreased; as a result, motorists may view a large change in direction as a motivation to slow their speed. In addi- tion, as curve length increased, speed measures increased, suggesting that drivers may become more comfortable at higher speeds because they have more time to adjust their vehicle path to a constant radius. Grade has an influ- ence on the upper-percentage range of vehicle speeds, because the 85th percentile speed decreased as approach grade increased (Schurr et al. 2002). • A study of driver behavior and errors on a selection of horizontal curves led Lyles and Taylor (2006) to conclude the following: – Drivers approaching curves routinely exceeded the posted speed limit as well as the posted advisory speed, where applicable. – Drivers had more errors at curves where they had limited or no visibility of the curves when the TCDs were first visible. – Drivers made more errors on horizontal curves that were adjacent to vertical curves, particularly crests that obscured a downstream horizontal curve. – There were increased errors when curves were com- bined with other elements, especially intersections. Vertical alignment • Current North American design practices might yield segments of the vertical curve where the driver’s view is constrained to a distance shorter than the required SSD. An alternative design procedure is recommended, based on a new model that incorporated longitudinal friction and acceleration, which produced new recom- mended values for minimum lengths of crest and sag vertical curves (Hassan 2004). • A weight/power ratio of 102 to 108 kg/kW (170 to 180 lb/hp) would be appropriate for freeways in Califor- nia and Colorado, and a weight/power ratio of 126 kg/kW (210 lb/hp) would be more appropriate in Pennsylvania, as compared with the 120 kg/kW (200 lb/hp) value rec- ommended in the 2001 Green Book (Torbic et al. 2005). • The upward divergent headlamp angle used in the sag curve design equation should be reduced from 1° to between 0.75° and 0.90° (Hawkins and Gogula 2008). allocation of Traveled Way Width • The benefits of 2+1 roads in Europe validated a recom- mendation for their use in the United States, to serve as an intermediate treatment between an alignment with periodic passing lanes and a full four-lane alignment. Such 2+1 roads are most suitable for level and rolling terrain, with installations to be considered on roadways with traffic flow rates of no more than 1,200 veh/hr in a single direction. The use of a cable barrier as a separa- tor is discouraged, and major intersections should be located in the buffer or transition areas between oppos- ing passing lanes, with the center lane used as a turning lane (Potts and Harwood 2003). • Passing activity on 2+1 roads was greatest at the begin- ning of the segments and the greatest benefits of decreased platooning and increased safety occurred within the first 0.9 mi of a passing lane segment (Gattis et al. 2006). • Most passing on Super 2 passing lanes occurs within the first mile of a passing lane, so additional length may be less useful than additional lanes in a Super 2 corridor, particularly at lower volumes. Designers should avoid intersections with state highways and high-volume county roads within passing lanes, consider terrain and right-of-way in determining alignment and placement of passing lanes, avoid the termination of passing lanes on uphill grades, and discourage passing lane lengths longer than 4 mi (Brewer et al. 2011). • Two-way left-turn lanes could be used as a strategy to reduce head-on collisions on two-lane roads (Neuman et al. 2003b). lane Width • There was no general indication that the use of lanes narrower than 12 ft on urban and suburban arterials increased crash frequencies. Geometric design policies should provide substantial flexibility for the use of lane widths narrower than 12 ft (Potts et al. 2007). • Lane widths of 11 or 12 ft provide optimal safety benefit for common values of total paved width on rural two-lane roads. Although 12-ft lanes appear to be the optimal

68 design for 26- to 32-ft total paved widths, 11-ft lanes perform equally well or better than 12-ft lanes for 34- to 36-ft total paved widths (Gross et al. 2009). road diet • The rate of road diet crashes occurring during the period after installation was about 6% lower than that of matched comparison sites. However, controlling for possible differential changes in average daily traffic, study period, and other factors indicated no significant effect of the treatment. Crash severity was virtually the same at road diets and comparison sites. Conversion to a road diet should be made on a case-by-case basis in which traffic flow, vehicle capacity, and safety are all considered (Huang et al. 2002). • The effects of the road diet on crashes in Iowa, account- ing for monthly crash data and estimated volumes for treatment and comparison sites, resulted in a 25.2% reduction in crash frequency per mile and an 18.8% reduction in crash rate (Pawlovich et al. 2006). Shoulder Width • For horizontal curves on two-lane nonresidential facili- ties that have 3 degrees of curvature, the width of the lane plus the paved shoulder should be at least 5.5 m (18 ft) throughout the length of the curve (Staplin et al. 2002). • Wider lane and shoulder widths are associated with a reduction in segment-related collisions on rural front- age road segments (Lord and Bonneson 2007). rumble Strips • Crashes at approximately 210 mi of undivided rural two-lane roads treated with centerline rumble strips were reduced by 14% and injury crashes were reduced by an estimated 15%. All frontal and opposing-direction sideswipe crashes were reduced by an estimated 21%, and those crashes involving injuries were reduced by an estimated 25%. All of the reductions were determined to be statistically significant (Persaud et al. 2003). • Crash data on roads treated with centerline rumble strips or shoulder rumble strips revealed noticeable crash reduc- tions on all classes of roads (rural and urban two-lane roads and freeways). Shoulder rumble strips placed as close to the edgeline as possible maximize safety ben- efits. The safety benefits of centerline rumble strips for roadways on horizontal curves and on tangent sections are for practical purposes the same (Torbic et al. 2009). Shoulder Edge Treatments • Plaxico et al. (2005) made the following recommenda- tions on design guidelines for using curbs on roadways with operating speeds greater than 60 km/h (37.3 mph): – Any combination of a sloping-faced curb that is 150 mm (6 in.) or shorter and a strong-post guardrail can be used where the curb is flush with the face of the guardrail up to an operating speed of 85 km/h. – Guardrails installed behind curbs are best not located closer than 2.5 m (8.2 ft) for any operating speed in excess of 60 km/h (37.3 mph). – For roadways with operating speeds of 70 km/h (43.5 mph) or less, guardrails may be used with sloping-face curbs no taller than 150 mm (6 in.) as long as the face of the guardrail is located at least 2.5 m (8.2 ft) behind the curb. – Where guardrails are installed behind curbs on roads with operating speeds between 71 and 85 km/h (44.1 and 52.8 mph), a lateral distance of at least 4 m (13.1 ft) is needed to allow the vehicle suspension to return to its pre-departure position. – At operating speeds greater than 85 km/h (52.8 mph), guardrails are used with 100-mm (4-in.) or shorter sloping-faced curbs, and are placed so that the curb is flush with the face of the guardrail. Operating speeds above 90 km/h (55.9 mph) require that the sloping face of the curb must be 1:3 or flatter and must be no more than 100 mm (4 in.) in height. • The “Safety Edge” treatment produced small but posi- tive results in crash reduction at 56 of 81 treated sites. For all two-lane highway study sites in two states, the best estimate of the treatment’s effectiveness was a reduction in total crashes of approximately 5.7%. The results were not statistically significant, but they were generally positive (Hallmark et al. 2006). roadside • Where possible at curb locations, provide a lateral offset to rigid objects of at least 6 ft from the face of the curb and maintain a minimum lateral offset of 4 ft (Dixon et al. 2008). • At lane merge locations, do not place rigid objects in an area that is 10 ft longitudinally from the taper point. The lateral offset for this 20-ft section is consistent with the lane width, typically 12 ft (Dixon et al. 2008). • A lateral offset of 6 ft from the curb face to rigid objects is preferred for higher-speed auxiliary lane locations, such as extended length right-turn lanes, and a 4-ft minimum lateral offset should be maintained (Dixon et al. 2008). • At locations where a sidewalk buffer is present, rigid objects are best not located in a buffer area with a width of 3 ft or less. For buffer widths greater than 3 ft, lat- eral offsets from the curb face to rigid objects are main- tained with a minimum offset of 4 ft. At these wider buffer locations, other frangible objects can be strate- gically located to help shield any rigid objects (Dixon et al. 2008). • Rigid objects are best not located in the proximity of driveways, and care should be taken to avoid placing

69 rigid objects on the immediate far side of a driveway. In addition, objects are not to be located within the required sight triangle for a driveway (Dixon et al. 2008). intersection alignment • Avoid approach grades to an intersection of greater than 6%. On higher design speed facilities (50 mph and greater), a maximum grade of 3% should be considered (Rodegerdts et al. 2004). • Avoid locating intersections along a horizontal curve of the intersecting road (Rodegerdts et al. 2004). • Strive for an intersection platform (including sidewalks) with a cross slope not exceeding 2%, as needed for accessibility (Rodegerdts et al. 2004). • Approach curvature can be used as a treatment to force a reduction in vehicle speed through the introduc- tion of horizontal deflection at high-speed intersection approaches, but it is discouraged at downhill approaches (Ray et al. 2008). • A skew angle greater than 20 degrees is not recom- mended in design when the design vehicle is a large vehicle or semitrailer (Son et al. 2002). • A minimum skew angle of 15 degrees will accommo- date age-related performance deficits at intersections where right-of-way is restricted (Staplin et al. 2002). auxiliary lanes • Adding left-turn lanes is effective in improving safety at signalized and unsignalized intersections, reducing crashes between 10% and 44% (Harwood et al. 2002). • Positive results can also be expected for right-turn lanes, with reductions in total intersection accidents between 4% and 14% (Harwood et al. 2002). • A method was developed to identify where installation of right-turn lanes at unsignalized intersections and major driveways would be cost-effective, indicating combinations of through-traffic volumes and right-turn volumes for which provision of a right-turn lane would be recommended. The economic analysis procedure can be applied by highway agencies using site-specific values for average daily traffic, turning volumes, acci- dent frequency, and construction cost for any specific location (or group of similar locations) of interest (Potts et al. 2007). modern roundabouts • A series of projects during the decade led to the pub- lication of two FHWA Informational Guides contain- ing recommendations and guidelines for all aspects of roundabout design. • General overarching principles of geometric design of roundabouts (Rodegerdts et al. 2010) include: – “Provide slow entry speeds and consistent speeds through the roundabout by using deflection. – Provide the appropriate number of lanes and lane assignment to achieve adequate capacity, lane volume balance, and lane continuity. – Provide smooth channelization that is intuitive to drivers and results in vehicles naturally using the intended lanes. – Provide adequate accommodation for the design vehicles. – Design to meet the needs of pedestrians and cyclists. – Provide appropriate sight distance and visibility for driver recognition of the intersection and conflicting users.” • Maximum entering design speeds are based on a theo- retical fastest path of 20 to 25 mph for single-lane round- abouts and 25 to 30 mph for multilane roundabouts (Rodegerdts et al. 2010). • Roundabout alignment is described as “optimally located when the centerlines of all approach legs pass through the center of the inscribed circle” (Robinson et al. 2000). • Common inscribed circle diameters for single-lane roundabouts vary from 90 to 180 ft, depending on design vehicle (Rodegerdts et al. 2010). • Designers “should provide no more than the minimum required intersection sight distance on each approach, [because] excessive intersection sight distance can lead to higher vehicle speeds that reduce the safety of the intersection for all road users” (Robinson et al. 2000). • Crash experience at selected intersections in the United States indicates an overall reduction in crash frequency at intersections converted to roundabouts (Rodegerdts et al. 2007). • Pedestrian refuge a minimum width of 6 ft will ade- quately provide shelter for persons pushing a stroller or walking a bicycle (Robinson et al. 2000). • At single-lane roundabouts, the pedestrian crossing is best located one vehicle-length (25 ft) away from the yield line. At double-lane roundabouts, the pedestrian crossing is best located one, two, or three car lengths (approximately 25 ft, 50 ft, or 75 ft) away from the yield line (Robinson et al. 2000). • The “pedestrian refuge should be designed at street level, rather than elevated to the height of the splitter island. This eliminates the need for ramps within the refuge area, which can be cumbersome for wheelchairs” (Robinson et al. 2000). • Ramps may be provided on each end of crosswalks to connect the crosswalk to other crosswalks around the roundabout and to the sidewalk network (Robinson et al. 2000). • A detectable warning surface, as recommended in the Americans with Disabilities Act Accessibility Guide- lines, may be applied to the surface of the refuge within the splitter island (Robinson et al. 2000). • Use of standard AASHTO island design for key dimen- sions, such as offset and nose radii, is encouraged. For

70 sidewalks, a setback distance of 5 ft, with a minimum of 2 ft is advised (Robinson et al. 2000). • For nonmotorized users such as bicyclists, one important consideration during the initial design stage is to main- tain or obtain adequate right-of-way outside the circula- tory roadway for the sidewalks. All nonmotorized users who are likely to use the sidewalk regularly, including bicyclists in situations where roundabouts are designed to provide bicycle access to sidewalks, should be con- sidered in the design of the sidewalk width. Recom- mended designs for single-lane roundabouts encourage bicycle users to merge into the general travel lanes and navigate the roundabout as a vehicle, explaining that the typical vehicle operating speed within the circula- tory roadway is in the range of 15 to 25 mph, which is similar to that of a bicycle (Rodegerdts et al. 2010). innovative intersection designs A number of new or innovative intersection designs were con- sidered during the decade; each of the following was described in one or more studies. • Displaced Left Turns showed considerable savings in average control delay and average queue length, as well as an increase in intersection capacity, in one series of microsimulation analyses (Hughes et al. 2010). • Median U-turns are typically a corridor treatment applied at signalized intersections but are also used at isolated intersections to alleviate specific traffic operational and safety problems (Hughes et al. 2010). • Median width of Restricted Crossing U-Turns is a crucial design element to accommodate large trucks without allowing vehicles to encroach on curbs or shoulders (Hughes et al. 2010). • Quadrant Roadways are best designed so that the left turn with the highest demand is the one that receives the most direct path (Hughes et al. 2010). • Double Crossover Intersections are found to have greater throughput than a conventional intersection, along with lower values for number of stops, average stop time per vehicle, average queue, and maximum queue length (Bared et al. 2005). • Arterial Interchanges have an overall capacity near 75% of a four-lane freeway (Eyler 2005). • J-Turn and Offset-T designs had reductions in crashes between 40% and 92% (Maze et al. 2010). • Two-Level Signalized Intersections produced modeled results with the shortest delay times in most evalua- tion scenarios as well as the least sensitivity to varia- tions in traffic volume compared with other innovative intersection types; however, delay increased when flow was unbalanced between the two crossing roads (Shin et al. 2008). • The additional right-of-way needed to construct each of these innovative designs was mentioned as a potential drawback by every report and author that addressed the issue of the intersection’s footprint. Pedestrian and Bicycle facilities at intersections • Suggested strategies (Raborn et al. 2008) for modifying intersections to accommodate bicycles and pedestrians included: – Reducing the crossing distance for bicyclists. – Realigning intersection approaches to reduce or eliminate intersection skew. – Modifying the geometry to facilitate bicycle move- ment at interchange on-ramps and off-ramps. – Providing refuge islands and raised medians. – Grade-separated crossings. • “Pedestrian facilities should be provided at all inter- sections in urban and suburban areas. In general, design of pedestrian facilities with the most challenged users in mind—pedestrians with mobility or visual impairments—should be done, and the resulting design will serve all pedestrians well. ADA requires that new and altered facilities constructed by, on behalf of, or for the use of State and local government entities be designed and constructed to be readily accessible to and usable by individuals with disabilities” (Rodegerdts et al. 2004). • Practitioners should incorporate key elements that affect a pedestrian facility into their design (Rodegerdts et al. 2004): – “Keep corners free of obstructions to provide enough room for pedestrians waiting to cross. – Maintain adequate lines of sight between drivers and pedestrians on the intersection corner and in the crosswalk. – Ensure curb ramps, transit stops (where applicable), pushbuttons, etc., are easily accessible and meet ADA Accessibility Guidelines design standards. – Clearly indicate the actions pedestrians are expected to take at crossing locations. – Design corner radii to ensure vehicles do not drive over the pedestrian area yet are able to maintain appropriate turning speeds. – Ensure crosswalks clearly indicate where crossings should occur and are in desirable locations. – Provide appropriate intervals for crossings and mini- mize wait time. – Limit exposure to conflicting traffic and provide ref- uges where necessary. – Ensure the crosswalk is a direct continuation of the pedestrian’s travel path. – Ensure the crossing is free of barriers, obstacles, and hazards.” Transit considerations • General intersection design principles and guidelines for transit issues (Eccles et al. 2007) include: – “Provide simple intersection designs. – Provide clear visual cues to make busway intersec- tions conspicuous. – Maximize driver and pedestrian expectancy. – Separate conflicting movements.

71 – Minimize street crossings. – Incorporate design features that improve safety for vulnerable users. – Coordinate geometric design features and traffic con- trol devices.” • There are four types of busways found at intersections: median busways, side-aligned busways, separated right- of-way busways, and bus-only ramps. Each busway type has unique characteristics that are considerations for guidance on safety issues, basic geometry (including placement of bus stops), and traffic control, along with examples of appropriate intersections for each type of busway (Eccles et al. 2007). access management at intersections • Right-turn-plus-U-turn could have better operational performance than direct left turns under certain traffic conditions, implying that directional median opening designs could provide more efficient traffic flow than full median openings (Zhou et al. 2002). • U-turns at signalized intersections resulted in a 1.8% saturation flow rate loss in the left-turn lane for every 10% increase in U-turn percentage and an additional 1.5% loss for every 10% U-turns if the U-turning move- ment was opposed by protected right-turn overlap from the cross street (Carter et al. 2005a). • Recommended practices (Potts et al. 2004) for rural unsignalized intersections include: – Medians that are as wide as practical, as long as the median is not so wide that approaching vehicles on the crossroad cannot see both roadways of the divided highway. – Where the AASHTO passenger car is used as the design vehicle, a minimum median width of 25 ft is recommended. – Where a large truck is used as the design vehicle, a median width of 70 to 100 ft generally is recom- mended. If such a median width cannot be provided, consideration should be given to providing a loon. • Recommended practices (Potts et al. 2004) for suburban unsignalized intersections include: – Median widths at suburban unsignalized intersections generally as narrow as possible while providing suffi- cient space in the median for the appropriate left-turn treatment. – Median widths between 14 and 24 ft will accommo- date left-turn lanes, but are not wide enough to store a crossing or turning vehicle in the median. – Medians wider than 25 ft may be used, but crossroad vehicles making turning and crossing maneuvers may stop on the median roadway. – Median widths of more than 50 ft generally should be avoided at suburban, unsignalized intersections. • Keep median opening lengths at rural divided high- way intersections generally to the minimum possible. Increases in median opening length are correlated with higher rates of undesirable driving behavior. In con- trast, the median opening in urban and suburban areas can be as long as necessary (Potts et al. 2004). interchange ramp design • The desired distance between the cross street and freeway merge point is at least 400 m (1,312 ft) for ramps at which metering is envisioned (Chaudhary and Messer 2002). • The source of the adjustment factors in the 2004 Green Book was provided in the 1954 AASHTO Blue Book, in which they first appeared as being based on applying “principles of mechanics to rates of speed change for level grades.” Further review did not reveal a procedure for determining adjustment factors. A new procedure contains an alternative set of adjustment factors for accel- eration length and deceleration length, the latter of which is based on the ratio of the deceleration length on a grade to the deceleration length on a level surface. Actual performance of vehicles on grades and on a level surface should be measured and compared with the sug- gested adjustment factors to determine the accuracy of those factors (Fitzpatrick and Zimmerman 2007). ramp and interchange Spacing • Recent guidelines make a distinction to separately define “ramp spacing” and “interchange spacing” and recom- mend ramp spacing values be the primary consider- ation in freeway and interchange planning and design (Ray et al. 2011). • Guidelines are presented based on four areas of emphasis: geometric design, traffic operations, signing, and safety. Geometric design principles, as well as site-specific fea- tures, dictate minimum lengths needed for ramps and other interchange components. Traffic volumes can necessitate increased spacing beyond the dimensions needed purely for geometrics. Safety tradeoffs, which have rarely been quantified until recently, can now be considered in project decision making. Finally, signing and other human fac- tors issues are best taken into account at the earliest in the evaluation process when making choices about ramp and interchange spacing (Ray et al. 2011). • Spacing assessments indicate that ramp spacing of less than 900 ft is likely not geometrically feasible. That spacing value increases up to 1,600 ft for entrance–exit ramp pairs (Ray et al. 2011). alternative interchange designs • Design practices for the Diverging Diamond interchange (Hughes et al. 2010) include: – “The minimum crossing angle of intersection should be 40 degrees. – The radius design should accommodate between 25 and 30 mph.

72 – Superelevation may not be needed because it could detract from any desired traffic calming effect. – Lane width should be around 15 ft. – Design should accommodate WB-67 trucks. – Adequate lighting should be provided. – Nearside signals should be considered. – Double Crossover Diamond interchange designs may only be appropriate where there are high-turning volumes. – Nearby intersections with high cycle lengths should be avoided. – Pedestrians at free-turning movements should be evaluated, and pedestrian signals may be needed. – The noses of the median island should extend beyond the off-ramp terminals to improve channelization and prevent erroneous maneuvers. – Left- and right-turn bays should be designed to allow for separate signal phases.” • The Displaced Left-Turn interchange has functions similar to a DLT at-grade intersection. DLT lanes typi- cally cross the opposing through traffic at locations that are approximately 400 to 500 ft upstream of the signal- controlled ramp terminals. Minimum median widths are preferred for this design (Hughes et al. 2010). BarriErS To WidESPrEad imPlEmEnTaTion This section discusses some potential barriers to the wide- spread implementation of the research and findings presented within the report. These potential barriers are presented as observations gleaned through the compilation of the material collected for this research. • A large number of the sources reviewed for this synthesis produced results and recommendations that incorporated the use of a series of complex equations and/or multiple assumptions to begin the analysis. Such complex meth- odology may not be conducive to practitioners because the complex equations do not facilitate their use or because the necessary data are not available. • Similarly, the use of computer-based simulation and modeling has greatly increased as technology improves. However, many designers, particularly those at agen- cies in smaller jurisdictions, do not have access to such software or expertise to successfully use it to obtain the results described in the research. • The advent of multiple innovative intersection treat- ments has led to a wide variety of potential outcomes, and the research to support those outcomes is not yet mature. Practitioners who desire to use one or more of these treatments are cautioned in multiple studies that results are still very preliminary. In addition, these treatments typically require additional right-of-way and construction costs. Although they may be less expen- sive than a fully grade-separated facility, the cost is still a major factor in determining which treatment to use. The added complexity of the design and the need to “train” drivers how to use the new intersections are also considerations. • As roadway agencies continue to investigate new ways to use their budgets more efficiently, the cost of any treatment will likely be further scrutinized, whether it is the realignment of a skewed intersection or the addi- tion of rumble strips to a lengthy section of two-lane highway. Treatments that can provide benefits at low costs would appear to become increasingly valuable and desirable in this fiscal environment. rEcommEndaTionS for furTHEr rESEarcH During the course of their projects, many researchers identi- fied gaps in knowledge or additional questions that were raised as a result of their findings. Other needs for future research have also been identified based on information that was not found within the literature that was reviewed for this synthesis report. Recommendations for research to fill those needs are summarized here: • Fitzpatrick et al. (2006) recommended that their findings on safety and operations at exclusive right-turn lanes be verified through use of a larger, more comprehensive study that includes right-turning volume. • Multiple studies mentioned the lack of data on U-turns at median openings not designed for U-turns and/or sug- gested this as a valid research topic to examine safety and operational effects of such maneuvers. • Carter et al. (2005a) discussed several potential research topics for U-turns at signalized intersections. Among them are potential benefits of “U-turn Must Yield” signs; mitigation of the effects of right-turn overlap; a U-turn prediction model based on driveway density, land usage, and other site characteristics; and the effects on capacity and safety of U-turning heavy vehicles. • NCHRP Report 672 (Rodegerdts et al. 2010) advised the use of a critical headway of 5.0 s, based on the criti- cal headway required for passenger cars. The authors added that this value represented an interim methodol- ogy pending further research. • With the advent and increasing popularity of electronic toll collection methods, toll plaza design practices are changing to a certain degree. Among the reviewed sources and the practitioners who focus on this area of geometric design, there appears to be a consensus that more recent information on updated practices may be fragmented, scattered, or not yet evaluated; a need exists for at least a compendium of the best knowledge currently available on those measures found to be the most successful in the application of geometric criteria in the design of fixed- barrier manually operated plazas as well as in the removal of the barriers and replacement with electronic open-road tolling gantries. • Research is needed on intermediate speeds in the range of 40 to 50 mph in urban and suburban areas, and their effects on various cross-sectional design elements. Such

73 cross-section elements include the allocation of lane and shoulder widths, use of various median types and widths, the provision of bike lanes, parking lanes, use of vertical or sloping curbs and gutters and associated offsets, clear zone widths, traffic barriers, utilities, and interactions of various combinations of these elements. • Highway designers are under increasing pressure to max- imize the use of available right-of-way in freeway corri- dors to provide safety, mobility, and capacity for growing traffic demand. With right-of-way limitations, increased use of context-sensitive designs, and implementation of managed facilities, designers must maximize the use of freeway cross sections. Although freeway cross-section design guidance suggests that 12-ft lanes with 8- to 10-ft inside and outside shoulders is ideal, there is limited research on how deviations from these ideals individu- ally, or in combination, will affect freeway operations and safety. Highway designers need guidance on the opera- tional and safety impacts for cross-section design trade- offs while trying to balance corridor capacity, project costs, public involvement, and environmental impacts. • In addition, there is concern over the part-time use of existing shoulders as high-occupancy vehicle, high- occupancy toll, or general-use facilities during peak hour. The trade-offs between operational benefits and safety need to be quantified. Further, the safety implications of violators using the shoulder during the off-peak period need to be quantified. It is unclear whether this changed view of the shoulder as part of the traveled way also transfers to shoulder violations on adjacent facilities. The signing and striping of these shoulders for clear communication of the changed cross section use must also be quantified. • Despite the many features of the Interactive Highway Safety Design Model, methods to assess design con- sistency for multi-lane rural highways and urban and suburban arterials are not available. Development of design consistency procedures for these facility types will provide a full suite of mobility and safety assess- ment tools for use by designers throughout the project development process. • Typically, ramp terminals and the ramp proper are designed independent of each other and the two com- ponents are simply put together in the final design of a ramp. Ramp design practices may consider driver expectations and behaviors over a full range of geomet- ric and traffic conditions that would include the inter- change form, ramp type, the area environment (rural vs. urban) and the functional classification of the two inter- changing roadways. The issue of an integrated ramp and ramp terminal design is a complex issue in need of basic research. • Decision sight distance policy is based on a relatively small research study completed for FHWA in 1978 (McGee et al.). Decision sight distance is clearly intended for application at selected locations where greater sight distance than SSD is needed. However, there is little practical guidance to help designers identify situations where decision sight distance is or is not appropriate. And, there is little available information on how decision sight distance criteria are actually being applied by high- way engineers and whether the decision sight distance policy is accomplishing its stated objective. • Traffic calming guidelines often discuss the benefits of designing roadways to improve pedestrian safety. In the- ory, roadways that are designed with certain characteris- tics can encourage slower motor vehicle speeds, which cause more motor vehicle drivers to yield to pedestrians crossing the street and result in less severe pedestrian injuries when crashes do occur. Yet, there is a lack of research that quantifies the complexity of relationship between the following three factors: (1) roadway design, (2) motor vehicle speed, and (3) motorist yielding behavior. The effects of roadway design treatments on driver yielding are unknown for many different combi- nations of traffic speed and roadway conditions. This makes it extremely difficult to craft pedestrian-oriented guidelines that are applicable to the wide range of con- ditions present in communities throughout the country. More research is needed to quantify how driver yielding behavior is related to travel speed and different roadway characteristics, such as lane widths, pavement condi- tions, horizontal and vertical shifts, sight distances, lateral clearance, and other factors. This research should be used to create improved guidelines for roadway design and traffic calming practice.