Evaluation of the 13 Controlling Criteria for Geometric Design (2014)

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42 S E C T I O N 3 This section of the report reviews current practices of highway agencies with respect to design exceptions. Although all exceptions from applicable design policies should be documented in some manner, as specified in the policies of individual highway agencies, formal design exceptions are required by federal policy only for projects on the NHS for which agencies seek exceptions to the 13 controlling criteria. Internal state agency policies may require formal design exceptions for additional design criteria or for non-NHS projects. To distinguish between these types of design exceptions, one state agency—the Georgia Department of Transportation—refers to excep- tions from federal policies as design exceptions and excep- tions from state policies as design variances. Virginia uses the term design waiver, rather than design variance, for devia- tions from state criteria. The information presented here is based both on published literature and on interviews with experienced designers. 3.1 Published Reviews of Design Exception Practices This section presents a summary of three published reviews of design exception practices—Mason and Mahoney (41), McGee et al. (42), and Stamatiadis et al. (43)—to com- plement the results of interviews with highway agency staff presented in the next portion of this section. 3.1.1 Mason and Mahoney NCHRP Synthesis 316: Design Exception Practices, by Mason and Mahoney (41), reviewed the design exception practices of state highway agencies, including the conduct of a survey on design exceptions to the controlling criteria to which 45 of the 50 states and the District of Columbia responded. Table 46 summarizes the controlling criteria that respondents indicated frequently required design exceptions. Design Exception Practices Mason and Mahoney found that the annual number of design exceptions prepared by state highway agencies ranges from 1 to approximately 500. Some of this variation is attrib- uted to the basic characteristics of the states, their road sys- tems, and their capital construction programs. The factors that an agency uses to determine if a design exception is needed are an additional source of variation in the numbers of design exceptions. These factors are • Project location/road system (considered by 28 percent of highway agencies) • Project funding source (13 percent) • Project scope/type (65 percent) • Supplemental agency design criteria (i.e., in addition to the 13 controlling criteria) (33 percent) • Agency design criteria values higher than AASHTO criteria (44 percent) • Use of established RRR criteria (87 percent) Six highway agencies noted that developing design excep- tion documentation is time and cost intensive; limited resources (agency personnel, funds, and time) may discour- age the use of design exceptions to achieve design flexibility for these agencies. Five agencies asked that FHWA clarify the controlling criteria and provide better guidelines; this request has been addressed by the subsequent FHWA publication, Mitigation Strategies for Design Exceptions (7). The following policy changes were requested by the individual state highway agencies interviewed: • Eliminate design speed as a controlling criterion • Revise the process for resurfacing (i.e., RRR projects) • Do not require design exceptions for existing features that do not meet current policy NCHRP Synthesis 316 recommended that the relation- ship of the controlling criteria to traffic operations and safety

43 should be reviewed to determine which relationships are strongest (41). This recommendation is addressed in this report. NCHRP Synthesis 316 also recommended that future research develop improved guidance for evaluating the safety implications of design exceptions. The publication of the first edition of the HSM (12) provides a tool that can be utilized for this purpose. However, specific guidance on how the HSM should be used in such analyses would be desirable. 3.1.2 McGee et al. McGee et al. (42) conducted a survey of state highway agen- cies concerning their practices for conducting safety analyses in connection with design projects. Responses were received from 37 of the 50 states. A summary of the responses from state highway agencies related to the types of design ele- ments frequently requiring design exceptions is presented in Table 47. 3.1.3 Stamatiadis et al. Stamatiadis et al. (43) prepared a paper reporting on their investigation of the design exception practices of the Kentucky Transportation Cabinet. They summarized past experience with design exceptions and found that design exceptions were requested for an average of 40 projects per year, often with more than one individual design exception per project. Over the 8-year period from 1993 to 2000, a total of 562 individual design exceptions, or 70 design exceptions per year, were requested. This experience is summarized in Table 48. The majority of the projects involved bridge replacement, with the next most frequent being reconstruc- tion for roadway widening or construction of turn lanes. As indicated in Table 48, the most common design excep- tion was for design speed lower than the posted speed limit. This contrasts with the experience reported by Mason and Mahoney (41), by McGee et al. (42), and by the interviews with highway agencies presented in Section 3.2, which indi- cated that design exceptions were generally not sought for design speed for an entire project, but rather for individual Table 46. Controlling criteria identified by state highway agencies as commonly requiring design exceptions (41). Design element Responses from state highway agencies Number of responses Percentage of responses Horizontal alignment 25 54 Shoulder width 24 52 Vertical alignment 20 43 Stopping sight distance 18 39 Lane width 12 26 Design speed 12 26 Superelevation 9 20 Bridge width 8 17 Grade 7 15 Horizontal clearance/lateral offset 7 15 Vertical clearance 3 7 Cross slope 1 2 Structural capacity 0 0 Table 47. Design elements identified by state highway agencies as frequently requiring design exceptions (42). Design element Responses from state highway agencies Number of responses Percentage of responses Shoulder width 21 57 Vertical alignment/curvature 12 32 Lane width 11 30 Horizontal alignment/curvature 11 30 Stopping sight distancea 7 19 Bridge width 6 16 Maximum grade 6 16 Clear zoneb 5 14 Sideslopeb 5 14 Lateral clearance 4 11 Superelevation 4 11 Reduced design speed 3 8 Existing bridge railb 2 5 Cross slope 2 5 Vertical clearance 2 5 a Includes only alignment-related design exceptions. b Not one of the 13 controlling criteria. Table 48. Number of design exceptions requested in Kentucky (1993 to 2000) (43). Design element Number of design exceptions Percentage of design exceptions Design speed 191 34.0 Horizontal alignment/curvature 67 11.9 Stopping sight distance 65 11.6 Shoulder width 63 11.2 Ditch widtha 43 7.7 Roadway width/lane width 42 7.5 Bridge width 35 6.2 Number of lanesa 16 2.8 Maximum grade 15 2.7 Superelevation 12 2.1 Acceleration lanea 4 0.7 Clear zone/borderb 3 0.5 Earth cut/fill slopea 2 0.4 Bridge railinga 1 0.2 Tie downa 1 0.2 Access spacinga 1 0.2 Guardrail end treatmenta 1 0.2 TOTAL 562 a Not one of the 13 controlling criteria. b Clear zone is not one of the 13 controlling criteria, but lateral offset (border) is a controlling criterion.

44 design elements (curve radius, superelevation, vertical curves, or lane width) where the designs specified in the controlling criteria could not be provided. Other common design exceptions were for stopping sight distance, curve radius, or shoulder width. Stamatiadis et al. conducted a crash analysis of 65 projects with design exceptions for which before and after data could be obtained. For 59 of the 65 projects, the crash rate following the project was (1) lower than the average crash rate for simi- lar sites or (2) lower than the crash rate at the same site before the project. However, this finding was based on a naïve before- after study that did not incorporate any compensation for the effects of regression to the mean. Relatively short before- and after-study periods were used because both periods, including an allowance for the “during construction” period, had to be fitted into the 6 years (1995 to 2000) for which data were avail- able. A closer examination of the six projects for which crashes increased found that the patterns of increased crashes were not related to the design elements covered by the design exception. The study concluded that the design exceptions being requested were reasonable, and there was no reason for the Kentucky Transportation Cabinet to change its design exception process. 3.2 Interviews with State Highway Agency Staff Interviews with state highway agency staff were conducted at highway agency offices, on the telephone, and at two national meetings: • AASHTO Technical Committee on Geometric Design, Irvine, California, July 25 through 27, 2011 • Highway Safety Manual Peer Exchange, convened as part of NCHRP Project 17-50, Irvine, California, August 10 and 11, 2011 An in-depth interview was conducted with highway agency design engineers in California. Brief interviews were conducted with engineers from the following states: Georgia, Kansas, Louisiana, Michigan, Minnesota, Missouri, Oregon, Tennessee, and Virginia. There were several common themes in the interview results for all states: • All agencies interviewed have a formal design exception process and a standard format for written documentation/ justification of design exceptions. • Design exceptions for design speed are seldom requested or granted. Where the design speed cannot be fully attained, it is more common to approve design exceptions for indi- vidual design elements than to change the overall design speed for the project. • Highway agencies are just beginning to consider how the HSM might be used in analysis of design exceptions. FHWA plans to address design exception analysis in a forthcoming HSM Applications Guide, but details of these plans are not yet known. • No highway agencies routinely do before-after evaluations of projects that included design, but some states are con- sidering doing this in the future. The results of these interviews are summarized below. 3.2.1 California Department of Transportation (Caltrans) Interviews were conducted on July 15, 2011, with project development coordinators and geometric design reviewers in the Caltrans Division of Design. Caltrans Highway Design Manual and Exception Process Caltrans publishes the Highway Design Manual (HDM). The HDM is a design manual for internal and external use and establishes uniform policies and procedures for the design of and designs on California highways. The manual consists of standards as well as techniques and reference materials. (It should be noted that Caltrans uses the word standards, in referring to the controlling criteria, while most other states avoid this term.) The manual is ever changing and expanding as the Division of Design releases new or amended standards and techniques. The standards contained in the manual fall into three categories: mandatory standards, advi- sory standards, and permissive standards. “Mandatory design standards are those considered the most essential to achieve overall design objectives.” (HDM 82.1[2]) “Advisory design standards are important also, but allow greater flexibility in application . . . ” (HDM 82.1[3]) Permissive standards are all other standards or recommendations contained in the manual and have no requirement of application. The FHWA 13 controlling criteria are all designated as mandatory stan- dards, except for the final standards of structural capacity, which are covered by Caltrans’ bridge design manuals. While the standards contained in the HDM are meant to promote uniform and safe design, for various reasons, it is sometimes difficult during project design to fully meet the standard. In cases where a mandatory or advisory standard can’t be met (and the project is under the jurisdiction of Cal- trans), a formal process of requesting an exception to the design standards must be followed. The Caltrans Division of Design is responsible for overseeing this process and for the approval or denial of all requests for exceptions to design stan- dards. Per the Caltrans Project Development and Procedures

45 Manual, “The purpose of the design exception process is to create a record that documents the engineering decisions lead- ing to the approval of each exception from a design standard.” The Relationship between Caltrans Policy and the 13 Controlling Criteria While the official design manual for the California high- way system is the HDM, there are many references within the HDM to FHWA/AASHTO documents that can be used as supplementary information. Specifically, the Green Book, and the RDG are commonly referred to in the HDM. Caltrans’ designers and geometric reviewers are currently utilizing the HSM, but not in an official capacity. The HSM contains CMFs that represent percentages of crash reduc- tion that could be expected upon implementation of a given countermeasure. Many project development coordinators and geometric reviewers find CRFs helpful when reviewing exception requests. Caltrans will likely be releasing an official policy on the validity and usage of the HSM and, specifically, CMFs. In fact, Caltrans is currently conducting research of its own to validate or invalidate CMFs based on California- specific crash data. The 13 controlling criteria adopted by FHWA are meant to apply to projects on the NHS. Many, if not most, of the man- datory and advisory standards found within the California HDM are based on the 13 controlling criteria. Basing its stan- dards on the 13 controlling criteria means that Caltrans has taken standards that apply to the NHS and extended them to all California state highway facilities. Furthermore, as many local agencies default to the HDM for their design standards, the 13 controlling criteria get carried further into the design of local roads. Often, Caltrans’ geometric reviewers consider the 13 con- trolling criteria when reviewing requests for exceptions to the mandatory/advisory standards, and some even go beyond the 13 controlling criteria back to the research data that support the criteria. Unfortunately, there are some highway engineers who are unaware of the existence of the 13 controlling criteria and, therefore, are unaware of the origins of the standards within the HDM. All too often, exceptions are requested due to political pres- sures and budget shortfalls and not enough consideration is given to the implications of implementing a lesser standard. The Caltrans Division of Design staff expressed great inter- est in the research conducted under NCHRP Project 17-53 and its potential ability to reaffirm and revalidate not only the 13 controlling criteria, but also standards in the HDM. This research could also serve to educate some that may be unaware of the origins of Caltrans’ standards. Though the majority of the mandatory/advisory standards found within the HDM can be traced back to 12 of the 13 controlling criteria, over the years many new standards have been added that do not relate directly to the FHWA criteria. Caltrans’ position is that some key issues of transportation design are not covered by the 13 controlling criteria and that the criteria could perhaps be expanded to include these issues. Caltrans staff members stated that the most commonly mentioned issue unrepresented by the 13 controlling criteria is access control. In the HDM, the term access control refers to how a public authority controls (whether partially or fully) access from adjoining lands to the highway system. At a time when high levels of development are surrounding aging high- way systems, access control has become a significant topic. The HDM also contains mandatory/advisory standards on pavement design, which the 13 controlling criteria do not address. The additional pavement design standards are based on a reasoning that longer pavement design life correlates to a higher level of worker safety and a lower level of interrup- tion to the traveling public. As evolving pavement designs provide for longer pavement life, the reduced frequency of maintenance means that fewer workers are in possibly unsafe environments. Less maintenance, of course, also means less traffic control and less traffic interruption. Caltrans does not believe it is necessary to include the pavement standards in the FHWA criteria. Caltrans also noted that the clear-zone concept is not included in the 13 controlling criteria. The clear-zone concept refers to the idea that having areas adjacent to the roadway be clear of any fixed objects is desirable for public safety. Studies show that on higher speed facilities a clear width of as much as 30 ft should be provided to allow the majority of errant vehicles to recover. This width makes a significant impact on a project, particularly when right-of-way is constrained. With such an impact and no mandatory standard in place, the clear zone provided is often below recommendations. The clear- zone concept is closely related to horizontal clearance, one of the 13 controlling criteria. The horizontal clearance criteria could possibly be expanded to include provisions for clear zone. Side slopes also are not mentioned in the 13 controlling criteria, but are certainly closely related to public safety and are part of the clear zone concept considerations. While the Caltrans list of mandatory and advisory stan- dards is growing, the list of most commonly requested design exceptions remains short and surprisingly does not vary sub- stantially by route designation. For conventional highways, the most common design exception request is for lane and shoulder widths. Many agen- cies have the view that wider facilities equal higher speeds. While it is increasingly perceived that providing less width for the traveling public will result in lower speeds, Caltrans believes there is actually no direct research to validate this perception. In rural areas, route consistency is often a moti- vating factor for the request. If a rural corridor has a limited

46 lane and/or shoulder width for its entire length, it may not make sense to upgrade a small section. Another factor in rural areas is environmental impacts. Most projects today require a “visual assessment” as part of the environmental studies and, in many rural areas, additional paved width (particularly full shoulders) is not considered desirable. On urban routes, dense development often means making existing widths work for a project design. This is usually done by reallocating the paved width to accommodate more lanes and/or bicycle facilities. For freeways, the most commonly requested exception is also for lane and shoulder widths, but it is usually the inside shoulder width and lane width for which Caltrans reviewers receive exception requests, as attempts are made to squeeze more capacity out of existing freeways. There are those in the industry who see some of the con- trolling criteria as interrelated and perhaps needing clarifica- tion. For example, design speed and horizontal alignment, as well as sight distance and vertical alignment (sag vertical curves) are closely related. However, the reviewers at Cal- trans are sufficiently experienced that they do not find these interrelations to be an issue. As the reviewers see it, a design speed that makes sense for a corridor should be set, and then any needed exceptions would be written for the alignment at spot locations. Rarely does Caltrans approve, or even receive, requests for exception to design speed for an entire project. Exceptions for sight distance in sag vertical curves rarely go unapproved, as this situation is easily mitigated with lighting. While some of the 13 controlling criteria continually receive design exceptions, there are also some criteria for which Caltrans rarely to never approves exceptions. Exceptions to the standard for vertical clearance to falsework are rarely approved as reviewers feel it is too important, and there are often collisions at structures during construction. Similarly, exceptions to structural capacity are never considered or required. There is a general consensus that structural capac- ity does not belong in the 13 controlling criteria and should be handled independently. 3.2.2 Georgia Department of Transportation An interview with Brent Story of the Georgia Department of Transportation (DOT) was conducted on July 26, 2011. The Georgia DOT approves approximately 120 design excep- tions and design variances per year; as noted above, the term design variance, rather than design exception, is used in cir- cumstances in which the federal policy on design exceptions does not apply. Georgia considers intersection skew angle as part of the controlling criteria for horizontal alignment, even though federal policy does not appear to require this. Georgia also treats intersection sight distance as a state controlling criterion; intersection sight triangles are documented graphi- cally on plan and profile sheets in an early design stage to confirm the availability of intersection sight distance. The most common design exceptions and design variances in Georgia are for inside shoulder widths on multilane road- ways and for lateral offset on local roads. In training design and design review staff, Georgia empha- sizes that a design exception or design variance is merely a mechanism to document a deviation from a criterion, but it should not be a roadblock to design flexibility. Thus, design exceptions and design variances are not discouraged where they make engineering sense. 3.2.3 Kansas Department of Transportation An interview with James Brewer of the Kansas DOT was conducted on July 25, 2011. The Kansas DOT approves only about three or four design exceptions per year. The Kansas DOT does its best to avoid the need for design exceptions. This is possible, perhaps, because of flatter terrain and fewer constraints than other states (at least in rural areas). The number of design exceptions might increase in the future if Kansas were to adopt a practical design philosophy like some other states, but they have not so far seen a need for this. The most common design exceptions have been for cross- section width, particularly, bridge width. It does not make sense to reconstruct or replace bridges that may be within a few inches of meeting established bridge width criteria. Very few excep- tions are needed for horizontal or vertical alignment. 3.2.4 Louisiana Department of Transportation and Development Terri Monaghan from the Louisiana Department of Trans- portation and Development (DOTD) safety staff made a pre- sentation and was interviewed at the HSM peer exchange on August 11, 2011. The Louisiana DOTD has adopted a policy that all statements about safety in design documents, includ- ing Stage 0 (Feasibility) documents, Stage 1 (Environmen- tal) documents, and design exceptions, must be quantified using HSM principles or other safety analyses, as applicable. Thus, the DOTD is now requiring HSM analyses as part of the design exception process. 3.2.5 Michigan Department of Transportation Brian Chomas of the Michigan DOT was interviewed on July 27, 2011. The Michigan DOT approves approximately 600 design exceptions per year. The most common design exceptions are for shoulder width, K values for vehicle curves (i.e., stopping sight distance), and acceleration/deceleration

47 lengths for freeway ramps. (The latter is a state criterion, rather than one of the 13 controlling criteria.) With respect to design speed, the FHWA Michigan Divi- sion Office allows design speeds equal to posted speeds, while the Michigan DOT prefers to use a design speed equal to the posted speed plus 5 mph. The Michigan DOT does not yet utilize the HSM in justify- ing design exceptions, but every design exception includes a crash analysis. 3.2.6 Minnesota Department of Transportation An interview with James Rosenow of the Minnesota DOT was conducted on July 27, 2011, and, as a follow-up, Mr. Rosenow provided a copy of the state policy on design standards and exceptions and the data shown in Table 49. The table shows the number of design exceptions requested by the Minnesota DOT during the years 2004 to 2010, inclusive. To keep the preparation of design exceptions from becom- ing too burdensome, the Minnesota DOT has developed sets of standard language that can be used, where appropriate, in documentation of design exceptions for particular design elements. This approach helps to keep documentation more uniform and to prevent the burden of preparing design exception documentation from becoming a barrier to the use of appropriate design exceptions. 3.2.7 Missouri Department of Transportation Jonathan Nelson from the Missouri DOT (MoDOT) safety staff made a presentation and was interviewed at the HSM peer exchange on August 11, 2011. MoDOT has made exten- sive use of the practical design philosophy; design exceptions are used to document and justify deviations from the control- ling criteria. In June 2011, MoDOT adopted a new policy in their Engineering Policy Guide that addresses safety analysis for design exceptions, incorporating requirements for use of both crash analysis and HSM analysis, as follows: If the design exception request involves any features that are safety related, then sufficient accident data and history is attached to the request to support the reasons for justification. A summary report of the accident information is acceptable if the volume of the data is excessive. Examples of safety related features are included in, but not limited to, the following list: lane width, shoulder width, shoulder type, rumble strips, turn lanes, bridge width, bridge approach rail, horizontal alignment, vertical alignment, grade horizontal clearance, vertical clearance, guardrail, etc. Any other items that may be perceived as a safety concern will also follow these requirements. In addition, if the design exception request involves safety related features that are adequately addressed in the AASHTO Highway Safety Manual, then documentation of the exception should include a safety analysis as described in the manual. In general, this safety analysis should compare the expected number of crashes for the facility with the design exception to the expected number of crashes of the facility without the design exception. Currently, not all safety related features are explicitly addressed in the High- way Safety Manual. A list of features currently addressed by the manual include: lane width, shoulder width, shoulder type, center line rumble strips, horizontal alignment (length, radius), grade roadside hazard rating, fixed objects, driveway density, median width sideslope, lighting, intersection skew angle and turn lanes. Not all features in the manual are addressed for each facility type. Since this policy is very new, there is little experience with it to date. 3.2.8 Oregon Department of Transportation Kent Belleque of the Oregon DOT was interviewed on July 27, 2011, and again on August 17, 2011. The Oregon DOT approves approximately 200 design exception elements per year (including RRR projects); multiple exceptions may be approved for a given project. The most common excep- tions are for shoulder width, lane width, and clear zones. (It should be noted that the latter item, clear zones, is not one of the 13 controlling criteria; however, Oregon has established the clear-zone guidelines of the AASHTO Roadside Design Guide (39, 40) as an internal policy for which design excep- tions are required.) Oregon has also added a 6-ft sidewalk width to the controlling criteria and seeks exceptions, where Design element Number of design exceptions Percentage of design exceptions Shoulder widtha 78 24.9 Bridge shoulder widthb 46 14.7 Horizontal alignment 36 11.5 Sag vertical curves 26 8.3 Normal cross slope 25 8.0 Lane width 22 7.0 Vertical clearance 22 7.0 Stopping sight distancec 17 5.4 Ramp lengthd 16 5.1 Grades 6 1.9 Type of bridge railingd 5 1.6 Lateral offset 4 1.3 Design speed 3 1.0 Crest vertical curves 3 1.0 Superelevation 3 1.0 Structural capacity 1 0.3 TOTAL 313 a Forty-one (41) design exceptions for right shoulder and 37 for left shoulder. b Twenty-three (23) design exceptions for right bridge shoulder and 23 for left bridge shoulder. c Horizontal stopping sight distance only. d Not one of the 13 controlling criteria. Table 49. Number of design exceptions requested by the Minnesota DOT (2004 to 2010).

48 needed, to utilize the minimum width of 5 ft in the U.S. Access Board’s proposed Public Rights of Way Guidelines (PROWAG). Oregon uses a centralized review committee to evaluate each proposed design exception prior to request- ing approval from the Chief Engineer. Legislation enacted at the request of the trucking industry requires review of design exceptions that would restrict trucks by a Motor Car- rier Committee. Oregon uses the practical design philosophy and has adopted a Practical Design Guide. Oregon does not yet use the HSM in justifying design exceptions. 3.2.9 Tennessee Department of Transportation An interview with Jeff Jones of the Tennessee DOT was con- ducted on July 26, 2011. The Tennessee DOT approves approxi- mately 10 to 12 design exceptions per year. Design exceptions might be used more often in Tennessee, but the onerous nature of the design exception process has become a barrier to the use of design exceptions. For example, FHWA is requiring design exception reports to include the cost of complying with the con- trolling criteria, even when this cost is very expensive to esti- mate and would be impractical to build. The most common design exceptions in Tennessee are for shoulder widths on existing roads originally built with design criteria lower than current criteria, including bridge widths, and vertical alignment, particularly crest vertical curves. 3.2.10 Virginia Department of Transportation Interviews were conducted with Bart Thrasher of the Virginia DOT (VDOT)on July 26, 2011, and with Theo- ron Knouse of VDOT on August 11, 2011. VDOT approves approximately 20 to 40 design exceptions per year, including both NHS and non-NHS facilities. The most common design exceptions are for shoulder width and horizontal alignment. (It should be kept in mind that VDOT maintains much of the local rural highway system that would be under county juris- diction in other states.) Virginia has begun using the HSM in design exception analyses. A VDOT representative stated that use of the HSM has modified the mitigation strategy for nearly every project to which it has been applied.