1.2.2 Design Waivers

When criteria are not met for non-controlling design elements, a design exception is not required. However, a departure from the minimum criteria outlined in the RDM will be handled by
design waivers
 at the District level. Design waivers will be granted as the District authorizes. A design waiver is not required when values meet or exceed the minimum requirements.
TxDOT’s non-controlling criteria for the following project categories will require a
design waiver
.
Minimum values requiring a design waiver for each of the non-controlling design criteria can be found in Appendix B.
1.2.2.1 New Location and Reconstruction Projects (4R)
Table 1-10
gives TxDOT’s non-controlling criteria for 4R projects that will require a
design waiver
 when the minimum requirements are not met. A design waiver
is not
 required when values meet or exceed the minimum requirements.
Table 1-10: TxDOT Non-Controlling Criteria for 4R Projects
Criteria
Minimum Criteria
Criteria Reference
Explanation
Curb Parking Lane Width
Appendix Table B-1
Section 4.10.16
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
Appendix Table B-2
Section 4.10.2 and 8.1.8
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
Appendix Table B-3
Section 4.10.2
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.
Curb Offset
Appendix Table B-4
Section 4.10.12
Minimum distance between the edge of the travel lane and the face of the curb.
Bridge Width
Appendix Table B-5
TxDOT Bridge Project Development Manual
Minimum overall bridge width.
Median Width
Appendix Table B-6
Section 4.10.15
Minimum width of separation between dual roadways which separates the traffic flow in the opposite direction.
Clear Zone
Appendix Table B-7
Section 4.10.7
The minimum unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles.
Lateral Offset to Obstructions
Appendix Table B-8
Section 4.10.8
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
Appendix Table B-9
TxDOT Bridge Project Development Manual
Minimum vertical and horizontal clearances.
Sag Vertical Curve Length
1
Appendix Table B-10
Section 4.8.2
Length of sag vertical curve.
Superelevation (Non-rate elements)
Appendix Table B-11
Sections 4.7.6 and 4.7.7
Non-rate elements include length and location of superelevation transitions.
Guardrail Length
Appendix Table B-12
Section 17.3
The Length of Need (LON) of guardrail required to protect both adjacent and opposing traffic, except where access accommodation is needed.
Shared Use Path Width
Appendix Table B-13
Section 18.4.1
Minimum width for Bikeways that are separated from vehicular traffic that also allow pedestrians and non-motorized users.
Separated Bike Lane/ Buffered Bike Lane Width
Appendix Table B-13
Sections 18.4.2 and 18.4.3
Minimum width for Bikeways that are separated from vehicular traffic by vertical elements or striped buffer areas in the street
Bike Accessible Shoulder Width
Appendix Table B-13
Section 18.4.6
Minimum width for Bikeways on a roadway adjacent to vehicle traffic.
Notes:
  1. Sag Vertical Curve Length applies to sag vertical curves for the purposes of a 4R Design Waiver. Sag Vertical Curve Length for vertical alignment is a direct correlation with the K-Value. If the minimum K-value is satisfied for a sag vertical curve (Figure 4- 7), then the Sag Vertical Curve Length is satisfied under usual conditions. For minimum Sag Vertical Curve Lengths at UnderCrossings, see Figure 4-29. See Section 4.8.2 for additional guidance on comfort criteria.
1.2.2.2 Resurfacing, Restoration or Rehabilitation Projects (3R)
Table 1-11
gives TXDOT’s non-controlling criteria for 3R projects that will require a
design waiver
 when the minimum requirements are not met.
A design waiver
is not
 required when values meet or exceed the minimum requirements.
Table 1-11: TxDOT’s Non-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 B-14
Section 10.1.5.2
Minimum design speed based on the functional classification.
Horizontal Curve Radius
X
Appendix Table B-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)
1
X
Appendix Table B-16
Section 4.11.1
Minimum SSD to be attained based on a vehicle’s ability to stop before reaching a stationary object in its path, at a certain design speed.
Maximum Grade
X
Appendix Table B-17
Section 4.8.1
Maximum gradient of a roadway along the centerline.
Superelevation (Non-rate elements)
X
X
Appendix Table B-18
Sections 4.7.6 and 4.7.7
Non-rate elements include length and location of superelevation transitions
Bridge Width
X
X
Appendix Table B-19
TxDOT Bridge Project Development Manual
Minimum overall bridge width.
Deficient Bridge Rail
X
Appendix Table B-20
TxDOT Bridge Railing Manual
Minimum requirements are met for new bridge rail or existing rail to remain in place.
Clear Zone
X
X
Appendix Table B-21
Section 4.10.7
The minimum unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles.
Speed Change Lane Width
X
X
Appendix Table B-22
Section 4.10.2
Minimum width of acceleration or deceleration lanes for left or right turns, exits or entrance lanes, or a climbing lane based on the highway class.
Length of Speed Change Lanes
X
X
Appendix Table B-23
Section 4.10.2
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.
Curbside Parking Lane Width
X
X
Appendix Table B-24
Section 4.10.16
Minimum width of parallel parking lanes based on the functional class of the roadway.
Guardrail Length
X
X
Appendix Table B-25
Section 17.6
The Length of Need (LON) of guardrail required to protect adjacent and opposing traffic, except where access accommodation is needed.
Sag Vertical Curve Length
2
X
X
Appendix Table B-26
Section 4.8.2
Length of sag vertical curve.
Notes:
  1. SSD only applies to crest vertical curves for the purposes of a Design Waiver for 3R low-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.
  2. Sag Vertical Curve Length applies to sag vertical curves for the purposes of a 3R Design Waiver. Sag Vertical Curve Length for vertical alignment is a direct correlation with the K-Value. If the minimum K-value is satisfied for a sag vertical curve (Figure 4-7), then the Sag Vertical Curve Length is satisfied under usual conditions. For minimum Sag Vertical Curve Lengths at Under-Crossings, see Figure 4- 29. See Section 4.8.2 for additional guidance on comfort criteria.
1.2.2.3 Resurfacing or Restoration Projects (2R)
Design waivers are not applicable to 2R projects.
1.2.2.4 Bicycle Facilities
Table 1-12
gives TxDOT’s non-controlling criteria for bicycle facility projects that will require a
design waiver
 when the minimum requirements given in
Chapter 18
cannot be met.
A design waiver
is not
 required when values meet or exceed the minimum requirements.
Table 1-12: TxDOT’s Non-Controlling Criteria for Bicycle Facilities
Criteria
Minimum Criteria
Criteria Reference
Explanation
Shared Use Path Width
Appendix Table B-13
Section 18.4.1
Minimum requirements based on context classification.
Separated Bike Lane/Buffered Bike Lane Width
Appendix Table B-13
Sections 18.4.2 and 18.4.3
Minimum requirements based on context classification.
Bike Accessible Shoulder Width
Appendix Table B-13
Section 18.4.6
Minimum requirements based on context classification.
1.2.2.5 Special Facilities
Table 1-13
gives TxDOT’s non-controlling criteria for off-system bridge replacement and rehabilitation projects with current ADT of 400 or less that will require a
design waiver
 when the minimum requirements are not met.
A design waiver
is not
 required when values meet the minimum requirements.
Table 1-13: TxDOT’s Non-Controlling Criteria for Off-System Bridges with ≤ 400 ADT
Criteria
Minimum Criteria
Criteria Reference
Explanation
Superelevation (Non-rate elements)
Appendix Table B-11
Section 12.2.2
Non-rate elements include length and location of superelevation transitions.
Sag Vertical Curve Length
1
Appendix Table B-10
Section 12.2.2
Length of sag vertical curve.
Bridge Width
Appendix Table B-5
Section 12.2.2
Minimum overall bridge width.
Notes:
  1. Sag Vertical Curve Length applies to sag vertical curves for the purposes of a 3R Design Waiver. Sag Vertical Curve Length for vertical alignment is a direct correlation with the k-value. If the minimum k-value is satisfied for a sag vertical curve (Figure 4- 7), then the Sag Vertical Curve Length is satisfied under usual conditions. For minimum Sag Vertical Curve Lengths at Under-Crossings, see Figure 4-29. See Section 4.8.2 for additional guidance on comfort criteria.
1.2.2.6 Off-System Historically Significant Bridge Projects
Design waivers
 are not applicable to off-system historically significant bridge projects.
1.2.2.7 Park Road Projects
Design waivers
 are not applicable to off-system park road projects. On-system Park Road projects need to meet the applicable criteria.
A Design Waiver form and other information can be found on the Design Division’s website (TxDOT intranet only).