Minnesota Department of Transportation

511 Travel Info

Frequently Asked Questions

Guardrail FAQs

End Treatments - Terminals

Yes. You will need to add a section of guardrail that transitions vertically between the existing 28" height guardrail and the new 31" end terminal. See Standard Plan 601.

For anchors near Bullnose Guardrail, recommended clearance is 20’.  See Standard Plan 688. 

For anchorages at median crossover, recommended clearance is 20’ from crossover.

For anchorages at signalized intersections coordinate with Maintenance and Traffic offices to determine the best location. These locations are unique and require context for determining optimal location.

For further information consult with Design Standards Unit.


Guardrail

Working width is defined as the distance between the barrier face before impact and the maximum lateral position of any major part of the system or vehicle during or after impact. This area should be free of any hazards or fixed objects. This detail (PDF) shows how working width is measured.

Yes. However, the approach guardrail transition (AGT) height for these barriers (See Standard Plans 603, 605, 607, 609, 618, and 619) are shorter, the Type 31 guardrail must transition vertically from 31” to 28” prior to connecting to the AGT. This 3" vertical transition requires a 25' horizontal transition. See Standard Plan 601 for the vertical transition.

Contact Design Standards if your project has need for a connection without a transition.

This depends on how far (laterally) off of the pavement the guardrail is located, and whether there is curb. This detail (pdf) shows the proper way to measure height of guardrail for different situations.

The Type 31 guardrail (Standard Plan 690) has been successfully tested with up to three consecutive posts omitted, leaving a maximum span of 25 feet between posts. Three CRT wood posts are required at either end of the span. Rail nesting is not required. See Standard Plan 696.

 

In addition, for Type 31 guardrail, omitting a single guardrail post is allowed once every 50’ without additional measures taken.

 

28" guardrail (Standard Plate 8338) can also have up to three consecutive posts omitted. Three CRT wood posts are required at either side of the span. The rail needs to be nested across the span as well as up- and down-stream from the gap, for a total length of 100 feet.

 

Note: Guardrail deflection distances/working widths increase when posts are omitted.

Standards have not yet been established for using Type 31 for short radius guardrail installations. Standard Plan 611 shows a bullnose transition to Type 31 guardrail.

Guardrail posts must be placed in soil that has a surface slope of 1V:10H or flatter. That slope can break to a 1V:2H or flatter slope with the slope break no less than 24 inches behind the guardrail post. Type 31 guardrail has been tested and approved for installation adjacent to a 1V:2H slope with the slope break at the back of the post when using posts that are 9 feet long. See Standard Plan 690.

A straightforward method to field verify LON in the field is to stand on the roadway edge directly opposite the shielded feature, and then pace off the appropriate runout length from the Table 10-7.03A in MnDOT’s Road Design Manual. At that point, turn and look at the shielded area. If the guardrail installation crosses that line of sight, then the area is adequately covered.

 

(If runout length values from the Table are unavailable pace off 300’ for high speed roadways or 200’ for low speed. If guardrail crosses this line of sight +/- 30’, the area is adequately covered.)

 

Note: This method is for field use only and should not be used when designing new or replacement systems.

 

Refer to this detail (pdf).

For Type 31 Guardrail (Standard Plan 690) and guardrail that is 28” in height, two 8" blockouts (16” maximum) may be used at any time for any number of posts. Three 8" blockouts, or two 12" blockouts (24" maximum) may be used at one post only once per 75’.

For Approach Guardrail Transitions (Standard Plans 693 and 934), additional blockouts are not acceptable with 12” blockouts. Two 8” blockouts may be used in limited applications depending on site condition. Consult with Design Standards.

Post bolt slots on standard w-beam rail are normally spaced at 3’ 1-1/2” (half post spacing). If the rail has no post bolt slots at ¼ post spacing the post with bolted blockout attached can be set flush behind the rail without drilling the slot in the guardrail.  Guard rail posts, blockouts, and post bolts through the rail are required at all full and half spaced rail locations.

Generally, a guardrail post can be set in a 1V:2H or flatter cut slope which allows for adequate post movement.

Guardrail can be installed adjacent to curb provided Standard Plan 690 is followed.

Wood posts should be replaced by steel posts on all new installations and reconstruction projects. See this technical memorandum (pdf) for guidance on preservation projects.


Transitions

The Approach Guardrail Transition (AGT) is a newer term for what has been historically called a Design Special by MnDOT. This new terminology better represents the system's purpose and also brings MnDOT in line with other States.

No, Standard Plate 8318 in not used with Type 31 AGT. Type 31 AGT shall be used on the approach end and the departing/trailing end of bridge rail/barrier if guardrail is needed.

No, the Type 31 Approach Guardrail Transition is currently available for the single slope barrier (Standard Plan 681) and vertical bridge end posts.

 

If a project requires a new transition and it has to connect to F or J barrier, use the appropriate approach guardrail transition ("Design Special" Standard Plans 603, 605, 607, 618) followed with either:

A) 28" guardrail (Standard Plate 8338) and an NCHRP 350 end terminal.

or

B) 25' vertical transition from 28" to 31" height guardrail (See Standard Plan 601) and a Type 31 (MASH) end terminal.

 

If the project requires you to replace the transition to a one line bridge rail, an end post should be built. (See Design Scene Chapter 14 for additional information (pdf))

Type 31 AGT shall be used on the departing/trailing end of bridge rail/barrier if guardrail is needed. See Standard Plans 693 and 694.