1.2.1 Design Exceptions

A design exception is required when the minimum values of the controlling criteria specified in this Chapter for the respective categories of construction projects are not met. A design exception is not required when values meet or exceed the minimum guidelines.
The controlling criteria for the following project categories will require a
design exception.
Minimum values requiring a design exception for each of the controlling design criteria can be found in Appendix A.
1.2.1.1 New Location and Reconstruction Projects (4R)
The adopted Federal Highway Administration (FHWA) design requirements in 23 CFR 625, and 49 CFR 37.9 apply to new location and reconstruction projects (4R) on the National Highway System (NHS).
Table 1-4
shows the list of AASHTO’s ten controlling design criteria that will require a
design exception
 when the minimum requirements specified in Appendix A are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
Table 1-4: AASHTO's Controlling Criteria for New Location and Reconstruction Projects (4R)
Criteria
Minimum Criteria
Criteria Reference
Explanation
Design Speed
Appendix Table A-1
Section 4.2
Minimum design speed based on the functional classification.
Lane Width
Appendix Table A-2
Note 1
Minimum lane width based on functional classification, type of lane (Mainlane, etc.) and design speed.
Shoulder Width
Appendix Table A-3
Note 2
Minimum outside and inside shoulder widths based on functional classification, type of facility and design speed.
Horizontal Curve Radius
Appendix Table A-4
Section 4.7.2
Minimum horizontal curve radius based on design speed and maximum superelevation rate.
Superelevation Rate
Table 4-4 through Table 4-7
Section 4.7.3
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)
3
Appendix Table A-5
Section 4.11.1
The minimum distance a driver 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
Appendix Table A-6
Section 4.8.1
Maximum gradient of a roadway along the centerline. For maximum grade for ramps see Table 15-2.
Cross Slope
Appendix Table A-7
Section 4.10.4
Maximum transverse slope across a pavement surface, excluding superelevation.
Vertical Clearance
Appendix Table A-8
Section 4.8.6
Minimum specified height of a bridge or overhead projection above the roadway.
Design Loading Structural Capacity
Appendix Table A-9
TxDOT Bridge Design Manual - LRFD
The Design Load Structural Capacity-Maximum Load that a bridge is designed to handle.
Notes:
  1. For Local Roads see Section 5.1.6; for Collectors see Section 6.1.6; for Arterials see Section 7.1.6; for Freeways see Sections 8.1.6; for Mobility Corridors see Section 9.2.2.
  2. For Local Roads see Section 5.1.7; for Collectors see Section 6.1.7; for Arterials see Section 7.1.7; for Freeways see Sections 8.1.7; for Mobility Corridors see Section 9.2.3.
  3. SSD applies to horizontal alignments and crest vertical curves for the purposes of a Design Exception. SSD for crest vertical curves is a direct correlation with the K-Value. If the minimum K-Value is satisfied for a crest vertical curve (Figure 4-6), then the vertical SSD is satisfied under usual conditions.
TxDOT implements additional controlling criteria for design elements that will require a
design exception
 if the conditions are not met on new location and reconstruction projects (4R).
Table 1-5
shows the list of TxDOT’s additional controlling criteria for new location and reconstruction projects that will require a
design exception
 when the minimum requirements are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
Table 1-5: TxDOT's Controlling Criteria for New Location and Reconstruction Projects (4R)
Criteria
Minimum Criteria
Criteria Reference
Explanation
Bridge Class Culvert Protection
Appendix Table A-10
TxDOT Bridge Railing Manual
All ADTs
Bridge Rail
Appendix Table A-11
TxDOT Bridge Railing Manual
Bridge rail must comply with MASH and meet requirements of the TxDOT Bridge Railing Manual.
1.2.1.2 Resurfacing, Restoration or Rehabilitation Projects (3R)
TxDOT implements
controlling criteria
 for the following design elements that will require a
design exception
 if the minimum conditions are not met on resurfacing, restoration, or rehabilitation projects (3R).
Table 1-6
shows the list of TxDOT’s additional controlling criteria that will require a
design exception
 when the minimum requirements are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
For 3R projects, “high-volume” roadways are those with greater than or equal to a current ADT of 1,500.
“Low-volume” roadways are defined as those with a current ADT of less than 1,500.
Table 1-6: TxDOT’s Controlling Criteria for Resurfacing, Restoration or Rehabilitation Projects (3R)
Criteria
High Volume ≥ 1500 ADT
Low Volume < 1500 ADT
Minimum Criteria
Criteria Reference
Explanation
Design Speed
X
Appendix Table A-12
Section 10.1.5.2
Minimum design speed based on the highway class and context.
Lane Width
X
X
Appendix Table A-13
Section 10.1.5.4
Minimum lane width based on the respective functional classification, context classification, and current Average Daily Traffic.
Shoulder Width
X
X
Appendix Table A-14
Section 10.1.5.4
Minimum shoulder width based on the respective functional classification, context classification, and current Average Daily Traffic.
Horizontal Curve Radius
X
Appendix Table A-15
Section 10.1.5.1
Minimum horizontal curve radius based on design speed and maximum superelevation rate.
Superelevation Rate
X
Table 4-4 through Table 4-7
Section 10.1.5.1
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)
3
X
Appendix Table A-16
Section 4.11.1
The minimum distance a vehicle needs to be able to see to have room to stop before colliding with something in the roadway. Use K-values to assist calculation.
Maximum Grade
X
Appendix Table A-17
Section 4.8.1
Maximum gradient of a roadway along the centerline.
Design Loading Structural Capacity
X
X
Appendix Table A-18
TxDOT Bridge Design Manual - LRFD
The Design Load Structural Capacity-Maximum Load that a bridge is designed to handle.
Bridge Class Culvert Protection
X
Appendix Table A-19
TxDOT Bridge Railing Manual
All ADTs
Deficient Bridge Rail
X
Appendix Table A-20
TxDOT Bridge Railing Manual
Minimum requirements are met for new bridge rail or existing rail to remain in place.
Notes:
  1. SSD applies to horizontal alignments and crest vertical curves for the purposes of a Design Exception for 3R high-volume roadways. SSD for crest vertical curves is a direct correlation with the K-Value. If the minimum K-Value is satisfied for a crest vertical curve (Figure 4-6), then the vertical SSD is satisfied under usual conditions.
1.2.1.3 Resurfacing or Restoration Projects (2R)
Design exceptions
 are required for 2R projects anytime the existing geometric or bridge features for the proposed project will be reduced below design standards.
A design exception
is not
 required when values meet or exceed the minimum requirements.
1.2.1.4 Bicycle Facilities
Table 1-7
gives the controlling criteria for bicycle facility projects that will require
a design exception
 when the minimum requirements are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
See
Section 18.1.7
for additional information on design exceptions for bicycle facilities.
Table 1-7: Controlling Criteria for Bicycle Facilities
Criteria
Minimum Criteria
Criteria Reference
Explanation
Bike Lane Width
Appendix Table A-21
Section 18.4.4.2
Minimum bike lane width requirement based on context classification.
Bike Shared Lane Width (Wide Outside Lane)
Appendix Table A-22
Section 18.4.7.2
Minimum lane width requirement based on context classification.
Bridge Deck Clear Space
Appendix Table A-23
Note 1
Minimum shoulder and offset measured to the toe of barrier provided on the structure.
Notes:
  1. For Local Roads see Section 5.1.7; for Collectors see Section 6.1.7; for Arterials see Section 7.1.7.
1.2.1.5 Special Facilities
Table 1-8
gives the requirements for off-system bridge replacement and rehabilitation projects with current ADT of 400 or less that will require a
design exception
 when the minimum requirements are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
If the current ADT is greater than 400, then applicable 4R or 3R criteria applies.
Table 1-8: TxDOT’s Controlling Criteria for Off-System Bridges with ≤ 400 ADT
Criteria
Minimum Criteria
Criteria Reference
Explanation
Design Speed
Appendix Table A-24
Section 12.2.2
Minimum design speed based on condition of the remainder of the road.
Stopping Sight Distance (SSD)
1
Minimum SSD to be attained based on the condition of the remainder of the road.
Horizontal Curve Radius
Minimum horizontal radius based on design speed and the condition of the remainder of the road.
Superelevation Rate
Minimum superelevation to be used based on the condition of the remainder of the road
Maximum Grades
Maximum grade to be used based on the condition of the remainder of the road.
Minimum Structure Width
Minimum width measured between face to face of rail depending on approach roadway width.
Bridge End Guard Fence
Minimum length of transition and end treatment use.
Approach Roadway Width
Minimum approach width, transition, and surfacing.
Design Loading Structural Capacity
The Design Load Structural Capacity-Maximum Load that a bridge is designed to handle.
Traffic Control
Traffic control for detours and placement of traffic control devices.
Notes:
  1. Stopping Sight Distance (SSD) applies to horizontal alignments and crest vertical curves for the purposes of a Design Exception. SSD for crest vertical curves is a direct correlation with the K-value. If the minimum K-Value is satisfied for a crest vertical curve (Figure 4-6), then the vertical SSD is satisfied under usual conditions.
1.2.1.6 Off-System Historically Significant Bridge Projects
Table 1-9
gives the controlling criteria for off-system historically significant bridge projects that will require a
design exception
 when the minimum requirements are not met.
A design exception
is not
 required when values meet or exceed the minimum requirements.
Table 1-9: TxDOT’s Controlling Criteria for Off-System Historically Significant Bridges
Criteria
ADT
Minimum Criteria
Criteria Reference
Explanation
Roadway Width
≤ 400
Appendix Table A-25
Section 12.3
Minimum roadway width based on the condition of the remainder of the road.
Load Carrying Capacity (Operating Rating)
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.
1.2.1.7 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 (TPWD).
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.
A design exception or design waiver
is not
 required when values meet or exceed the minimum requirements.
A Design Exception form and other information can be found on the Design Division’s website (TxDOT intranet only).