Section 2: Design Exceptions, Design Waivers, Design Variances, and Texas Highway Freight Network (THFN) Design Deviations
Anchor: #i1002889Overview
This subsection discusses the following topics:
- Anchor: #BLDNAIDL
- Design Exceptions; Anchor: #JFMUBGFE
- Design Waivers; Anchor: #LGQPRUNO
- Design Variances; and Anchor: #DVJWQXRA
- Texas Highway Freight Network (THFN) Design Deviations.
Design Exceptions
A design exception is required whenever the controlling criteria specified for different categories of construction projects (i.e., 4R, 3R, 2R, Special Facilities, Off-System Bridge Replacement and Rehabilitation projects, Historically Significant Bridge Projects, On-System Park Road Projects, and on-street Bicycle Facilities) are not met. Approval of a design exception rests with the TxDOT District, unless the project is subject to federal oversight or review. A design exception is not required when values exceed the minimum guidelines for the controlling criteria.
As per the Stewardship and Oversight Agreement on Project Assumption and Program Oversight by and Between Federal Highway Administration, Texas Division and the State of Texas Department of Transportation, 2015 (FHWA-TxDOT S&O Agreement, 2015), the FHWA has retained approval of design exceptions on the Interstate. Additionally, if a project (Interstate or other) is identified on the FHWA Texas Division’s list of “Involved Projects” (TxDIP), and the project’s TxDIP plan identifies any design exception responsibility to FHWA, then all proposed design exceptions for that project will be required to be submitted to FHWA. Design exceptions to be submitted to the FHWA, must first be submitted to the Design Division - Project Development Support Section for review; the Design Division - Project Development Support Section will then transmit the design exception to the FHWA for approval. All other project design exceptions are to be approved by the TxDOT District.
The FHWA adopted design requirements in 23 CFR 625, and 49 CFR 37.9 apply to new and reconstruction projects on the NHS. Note, in some instances TxDOT RDM standards are more restrictive than FHWA adopted standards. In cases where FHWA adopted standards are met but TxDOT standards are not met, an FHWA approved design exception is not required and the requests should be processed as an internal TxDOT design exception. For example, an FHWA approved design exception would be required if a minimum 16.0 ft. vertical clearance on an Interstate was not met (as listed in 23 CFR 625.4); since TxDOT has a 16.5 ft. vertical clearance requirement for an Interstate (that does not meet the project criteria for the THFN vertical clearance requirements) then an Interstate vertical clearance of between 16.0 ft. and less than 16.5 ft. would require an internal TxDOT design exception. Additionally, a vertical clearance of less than 16 ft. on the Interstate requires DoD coordination and approval. Vertical clearance coordination is required with the Surface Deployment and Distribution Command Transportation Engineering Agency (SDDCTEA). A form is available through TxDOT Design Division and must be included with the Design Exception approval request to FHWA.
Design exceptions involving the design loading structural capacity or bridge rails must be sent to the Bridge Division for their review and approval.
All design exceptions must be signed by the TxDOT District Engineer and this signature authority cannot be delegated. Design exceptions must be documented on the Form 1002.
The review of design exceptions and recommendations for approval/non-approval may be established individually by each TxDOT District. For example, a four person review committee might be established which would include:
- Anchor: #HCTJWGBO
- Director of Transportation Planning and Development; Anchor: #EEAXXGIC
- Director of Construction; Anchor: #BCBDHDYK
- Director of Operations/Traffic; and Anchor: #FMKQIKNC
- Area Engineer (not responsible for project management).
The recommendation of approval/non-approval by a majority of the committee members would constitute a quorum for recommending signature action.
The complete documentation for a roadway design exception should be retained permanently in the District project files. Since the construction plans are sealed, the design exception documentation does not require an engineer’s seal.
The following project categories will have controlling criteria that dictate a design exception.
New Location and Reconstruction Projects (4R)
Table 1-1 gives the controlling criteria for 4R projects that will require a design exception.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Design Speed |
Table 3-1 Table 3-5 Table 3-7 Table 3-11 Table 3-17 |
Minimum design speed based on the functional classification. |
|
Lane Width |
Table 3-1 Table 3-5 Table 3-8 Table 3-11 Table 3-18 |
Minimum lane width based on functional classification, type of lane (Mainlane, Speed Change Lane, etc.) and design speed. |
|
Shoulder Width |
Table 3-1 Table 3-5 Table 3-8 Table 3-11 Table 3-18 |
Minimum outside and inside shoulder widths based on functional classification, type of facility and design speed. |
|
Horizontal Curve Radius |
Table 2-3 Table 2-4 Table 2-5 |
Minimum horizontal curve radius based on design speed and maximum superelevation rate. |
|
Superelevation Rate |
Table 2-3 Table 2-4 Table 2-5 |
The amount of cross slope needed on a horizontal curve to help counterbalance the centrifugal force of a vehicle traversing the curve based on the design speed. |
|
Stopping Sight Distance (SSD)1 |
Table 2-1 |
The distance a vehicle needs to be able to see in order to have room to stop before colliding with something in the roadway. Use k-values to assist calculation. |
|
Maximum Grade |
Table 2-9 |
Maximum gradient of a roadway along the centerline. |
|
Cross Slope |
Chapter 2 – Pavement Cross Slope |
Transverse slope across a pavement surface. |
|
Vertical Clearance |
Table 2-11 |
Minimum specified height of a bridge or overhead projection above the roadway. |
|
Design Loading Structural Capacity |
Bridge Inspection Manual (Ch. 5, Section 3) |
The Design Load Structural Capacity-Maximum Load that a bridge is designed to handle. |
|
Bridge Class Culvert Protection |
TxDOT Bridge Railing Manual, RDM Ch. 2, Section 8. |
Protection of bridge class culvert is not provided for ADT > 1,500. |
|
Bridge Rail |
TxDOT Bridge Railing Manual Ch. 4, Section 2. |
Bridge rail must comply with mash and meet requirements in Ch. 4, Section 2 of the TxDOT Bridge Railing Manual. |
|
Note:
|
||
Resurfacing, Restoration or Rehabilitation (3R) Projects
Table 1-2 gives the controlling criteria for 3R projects that will require a design exception. For 3R projects, “high-volume” roadways are defined as current ADT of 1,500 and greater.
|
Criteria |
Low Volume |
High Volume |
Reference |
Explanation |
|---|---|---|---|---|
|
Deficient Bridge Rails |
X |
TxDOT Bridge Railing Manual |
Minimum requirements are met for new bridge rail or existing rail to remain in place. |
|
|
Design Speed |
X |
Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 |
Minimum design speed based on the highway class and context. |
|
|
Horizontal Curve Radius |
X |
Table 2-3 Table 2-4 Table 2-5 |
Minimum horizontal curve radius based on design speed and maximum superelevation rate. |
|
|
Superelevation Rate |
X |
Table 2-3 Table 2-4 Table 2-5 |
The amount of cross slope needed on a horizontal curve to help counterbalance the centrifugal force of a vehicle traversing the curve based on the design speed. |
|
|
Stopping Sight Distance (SSD)1 |
X |
Table 2-1 |
The distance a vehicle needs to be able to see in order to have room to stop before colliding with something in the roadway. Use k-values to assist calculation. |
|
|
Maximum Grade |
X |
Table 2-9 |
Maximum gradient of a roadway along the centerline. |
|
|
Lane Width |
X |
X |
Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 |
Minimum lane width based on the respective Functional Classification, highway class, and Current Average Daily Traffic. |
|
Shoulder Width |
X |
X |
Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 |
Minimum shoulder width based on the respective Functional Classification, highway class, and Current Average Daily Traffic. |
|
Design Loading Structural Capacity |
X |
X |
Bridge Inspection Manual (Ch. 5, Section 3) |
The Design Load Structural Capacity-Maximum Load that a bridge is designed to handle. |
|
Note:
|
||||
Resurfacing or Restoration Projects (2R)
Design exceptions are required for 2R projects any time the existing geometric or bridge features for the proposed project will be reduced below design standards.
Bicycle Facilities
Table 1-3 gives the controlling criteria for bicycle facility projects that will require a design exception when the minimum requirements given in AASHTO’s Guide for the Development of Bicycle Facilities cannot be met. See Chapter 6, Section 4, Bicycle Facilities for additional information.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Bike Lane |
Chapter 6, Section 4, Bicycle Facilities |
Urban and Rural criteria given. |
|
Shared Lane (Wide Outside Lane) |
Chapter 6, Section 4, Bicycle Facilities |
Urban and Rural criteria given. |
|
Bridge Deck Clear Space |
Chapter 6, Section 4, Bicycle Facilities |
Minimum shoulder and offset measured to the toe of barrier provided on the structure. |
Special Facilities
Table 1-4 gives the requirements for off-system bridge replacement and rehabilitation projects with current ADT of 400 or less that will require a design exception. If the current ADT is greater than 400, then applicable 4R or 3R criteria applies. See Chapter 6 Section 1, Off-System Bridge Replacement and Rehabilitation Projects.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Design Speed |
Chapter 6, Section 1, Off-System Bridges |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Approach Roadway Width |
Chapter 6, Section 1, Off-System Bridges |
Minimum approach width, transition, and surfacing. |
|
Bridge End Guard Fence |
Chapter 6, Section 1, Off-System Bridges |
Minimum treatment required. |
|
Horizontal Curve Radius |
Chapter 6, Section 1, Off-System Bridges |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Superelevation Rate |
Chapter 6, Section 1, Off-System Bridges |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Stopping Sight Distance (SSD)1 |
Table 2-1 |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Maximum Grade |
Chapter 6, Section 1, Off-System Bridges |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Design Loading Structural Capacity |
Bridge Inspection Manual (Ch. 5, Section 3) |
The Design Load Structural Capacity- Maximum Load that a bridge is designed to handle. |
|
Note:
|
||
Off-System Historically Significant Bridge Projects
Table 1-5 gives the controlling criteria for off-system historically significant bridge projects that will require a design exception. See Chapter 6 Section 1, Off-System Bridge Replacement and Rehabilitation Projects.
|
Criteria |
ADT |
Reference |
Explanation |
|---|---|---|---|
|
Roadway Width |
≤ 400 |
Chapter 6, Section 1, Off-System Bridges |
Meet or improve conditions that are typical on the adjacent sections of roadway. |
|
Load Carrying Capacity (Operating Rating) |
Bridge Inspection Manual (Ch. 5, Section 3) |
Operating Rating, the maximum permissible live load that can be placed on the bridge. This load rating also includes the same load in multiple lanes. Allowing unlimited usage at the Operating Rating level will reduce the life of the bridge. |
Park Road Projects
Design exceptions are not applicable to park road projects that are off the state highway system and designated as Park and Wildlife Roads (PW). Design is based on the criteria and guidance given in the current publication of the Texas Parks and Wildlife Department Design Standards for Roads and Parking, or as approved by the Texas Parks and Wildlife Department.
On-system park road projects, designated as Park Roads (PR), must meet the required design criteria for the appropriate roadway classification including design exception or design waiver requirements.
Anchor: #i1003162Design Waivers
When the criterion is not met in a non-controlling category, a design exception is not required. However, a departure from the minimum criteria will be handled by design waivers at the District level. Design waivers will be granted as the District authorizes. Complete documentation should be retained permanently in the District project files. On system Park Road projects need to meet the applicable criteria, see Chapter 6, Section 3 for additional information.
The non-controlling criteria for the following project categories will require a design waiver.
New Location and Reconstruction Projects (4R)
Table 1-6 gives the non-controlling criteria for 4R projects that will require a design waiver.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Curb Parking Lane Width |
Table 3-1 Table 3-5 |
Minimum width required for vehicles to park on the edge of the roadway in urban and suburban areas based on the functional class of the roadway. |
|
Speed Change Lane Width |
Table 3-1 Table 3-2 Table 3-5 Table 3-18 |
Minimum width of acceleration or deceleration lanes for left or right turns, exit or entrance lanes, or a climbing lane based on the highway class. |
|
Length of Speed Change Lanes |
Table 3-3 Table 3-4 Table 3-12 Table 3-13 Table 3-18 Table 3-23 Figure 3-44 |
Minimum length of acceleration or deceleration lanes for left or right turns, exit or entrance lanes, or a climbing lane based on the highway class. Minimum Spacing between ramps as shown in Figure 3-44, or acceptable LOS not met. |
|
Curb Offset |
Table 3-1 Table 3-5 |
Minimum distance between the edge of the travel lane and the face of the curb. |
|
Bridge Width |
TxDOT Bridge Project Development Manual |
Minimum overall bridge width. |
|
Median Opening Width |
Table 3-1 Table 3-2 Table 3-5 Table 3-11 |
Minimum width of separation between dual roadways which separates the traffic flow in the opposite direction. |
|
Clear Zone |
Table 2-12 |
The unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. |
|
Lateral Offset to Obstructions |
Chapter 2, Lateral Offset to Obstructions |
Minimum distance from the edge of the traveled way, beyond which a roadside object will not be perceived as an obstacle and result in a motorist’s reducing speed or changing vehicle position on the roadway. |
|
Railroad Overpass Geometrics |
TxDOT Bridge Project Development Manual |
Minimum vertical and horizontal clearances. |
|
Sag Vertical Curve Length1 |
Figure 2-1 Figure 2-7 |
Length of sag vertical curve. |
|
Superelevation (Non-rate elements) |
Chapter 2, Section 5 Horizontal Alignment |
Non-rate elements include length and location of superelevation transitions. |
|
Guardrail Length (unless for access accommodation) |
Appendix A, Longitudinal Barriers |
The Length of Need (LON) of guardrail required to protect traffic adjacent to guardrail, as well as apposing traffic. |
|
Shared Use Path |
Chapter 6, Section 4, Bicycle Facilities |
Bikeways that are physically separated from vehicular traffic and allows pedestrians and other non-motorized users. |
|
Bridge Class Culvert Protection |
TxDOT Bridge Railing Manual, and RDM Ch. 2, Section 8 |
Protection of bridge class culvert is not provided for ADT ≤ 1500. |
|
Note:
|
||
Resurfacing, Restoration or Rehabilitation (3R) Projects
Table 1-7 gives the non-controlling criteria for 3R projects that will require a design waiver. For 3R projects, “low-volume” roadways are defined as a current ADT of less than 1500.
|
Criteria |
Low Volume |
High Volume |
Reference |
Explanation |
|---|---|---|---|---|
|
Design Speed |
X |
Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 |
Minimum design speed based on the functional classification. |
|
|
Horizontal Curve Radius |
X |
Table 2-3 Table 2-4 Table 2-5 |
Minimum horizontal curve radius based on design speed and maximum superelevation rate. |
|
|
Superelevation Rate |
X |
Table 2-3 Table 2-4 Table 2-5 |
The amount of cross slope needed on a horizontal curve to help counterbalance the centrifugal force of a vehicle traversing the curve based on the design speed. |
|
|
Stopping Sight Distance (SSD)1 |
X |
Table 2-1 |
The distance a vehicle needs to be able to see in order to have room to stop before colliding with something in the roadway. Use k-values to assist calculation. |
|
|
Maximum Grade |
X |
Table 2-9 |
Maximum gradient of a roadway along the centerline. |
|
|
Superelevation (Non-rate elements) |
X |
X |
Chapter 2, Section 5 Horizontal Alignment |
Non-rate elements include length and location of superelevation transitions |
|
Bridge Width |
X |
X |
TxDOT Bridge Project Development Manual |
Minimum overall bridge width. |
|
Deficient Bridge Rails |
X |
TxDOT Bridge Railing Manual |
Minimum requirements are met for new bridge rail or existing rail to remain in place. |
|
|
Clear Zone |
X |
X |
Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 |
The unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. |
|
Turn Lane Width |
X |
X |
Table 4-1 Table 4-2 Table 4-3 |
Minimum width of speed change lanes. |
|
Length of Speed Change Lanes |
X |
X |
Table 3-3 Table 3-4 Table 3-12 Table 3-13 Figure 3-44 |
Minimum length of acceleration or deceleration lanes for left or right turns, exit or entrance lanes, or a climbing lane based on the highway class. Minimum Spacing between ramps as shown in Figure 3-44, or acceptable LOS not met. |
|
Curbside Parking Lane Width (Parallel Parking) |
X |
X |
Table 4-3 |
Minimum width of parallel parking lanes based on the functional class of the roadway. |
|
Guardrail Length (unless for access accommodation) |
X |
X |
Appendix A, Longitudinal Barriers |
The Length of Need (LON) of guardrail required to protect traffic adjacent to guardrail, as well as apposing traffic. |
|
Sag Vertical Curve Length 2 |
X |
X |
Figure 2-1 Figure 2-7 |
Length of sag vertical curve. |
|
Shared Use Path |
X |
X |
Chapter 6, Section 4, Bicycle Facilities |
Bikeways that are physically separated from vehicular traffic and allows pedestrians and other non-motorized users. |
|
Notes:
|
||||
Resurfacing or Restoration Projects (2R)
Design waivers are not applicable to 2R projects.
Bicycle Facilities
Table 1-8 gives the non-controlling criteria for bicycle facility projects that will require a design waiver when the minimum requirements given in AASHTO's Guide for the Development of Bicycle Facilities cannot be met. See Chapter 6, Section 4, Bicycle Facilities.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Shared Use Path (Independent alignment or Side Path) |
Chapter 6, Section 4, Bicycle Facilities |
Urban and Rural criteria given. |
|
Separated Bike Lane/Buffered Bike Lane |
Chapter 6, Section 4, Bicycle Facilities |
Urban and Rural criteria given. |
|
Bike Accessible Shoulder Width |
Chapter 6, Section 4, Bicycle Facilities |
Urban and Rural criteria given. |
Special Facilities
Table 1-9 gives the non-controlling criteria for off-system bridge replacement and rehabilitation projects with current ADT of 400 or less that will require a design waiver.
|
Criteria |
Reference |
Explanation |
|---|---|---|
|
Superelevation (Non-rate elements) |
Chapter 2, Section 5, Horizontal Alignment |
Superelevation (Non-rate elements) |
|
Sag Vertical Curve Length1 |
Figure 2-1 Figure 2-7 |
Length of sag vertical curve. |
|
Bridge Width |
Chapter 6, Section 1, Off-System Bridges |
Minimum Structure Width, see Chapter 6 Section 1 for criteria. |
|
Note:
|
||
Off-System Historically Significant Bridge Projects
Design waivers are not applicable to off-system historically significant bridge projects.
Park Road Projects
Design waivers are not applicable to or off-system park road projects.
Anchor: #i1003307Design Variances
TxDOT designers must use PROWAG to achieve accessible design requirements in the public right-of-way. TAS and DOJ ADA Standards (2010) must be used for design and construction of buildings and sites such as TxDOT buildings, Safety Rest Areas, etc. A request for a design variance for any deviations from the applicable PROWAG, or if applicable, TAS requirements must be submitted to the TDLR for approval. Design variances should be sent to the Design Division – Landscape Architecture Section for review prior to being submitted to TDLR for approval. Refer to Chapter 7, Section 3, Pedestrian Facilities for additional discussion. This documentation should be kept in the project files.
A Design Variance form, process flowchart, and other information can be found on the Design Division's website.
Anchor: #i1019945Texas Highway Freight Network (THFN) Design Deviations
A Texas Highway Freight Network (THFN) design deviation is required for projects that do not meet specified Bridge Vertical Clearance Requirements; see Chapter 3, Section 8 for applicable project criteria, and Table 2-11 for vertical clearance values. The Deviation request is exclusive of (and in addition to) the design exception that may be needed for Vertical Clearances not meeting the specified requirements in Chapter 3 of the respective roadway facility. The THFN Deviation requests must be submitted to the Design Division – Project Development Support Section, and will be reviewed by an assigned Design Deviation Committee.
A Design Deviation form, process flowchart, and other information can be found on the Design Division’s website.