57

APPENDIX A. USMP RATING FORM DETAILED DESCRIPTIONS

UNSTABLE SLOPE MANAGEMENT

PROGRAM FOR FEDERAL LAND

MANAGEMENT AGENCIES –

RATING CATEGORY 

DESCRIPTIONS

Landslide Technology, WFLHD, NPS,

USFS, BLM, and Bureau of Indian Affairs (BIA)
 

58

TABLE OF CONTENTS

SITE INFORMATION ...................................................................................59

PRELIMINARY RATINGS .............................................................................65

DETAILED HAZARD RATINGS ......................................................................76

DETAILED RISK RATINGS (ROCKFALL AND LANDSLIDES).............................88

REFERENCES FOR APPENDIX A ...................................................................95
59

SITE INFORMATION

The top of the field data collection sheet contains fields for the collection of location and site 

information. These fields record location information and field measurements critical for later 

rating criteria. Many of the fields should be self-evident to the geological or engineering 

personnel who will be providing training, overseeing, and possibly performing the ratings, but 

some of the fields are explained in more detail below. Heading order follows the rating form

(see sample slope rating form in Appendix B).

Management Area

Federal Land Management Agency (FLMA) management area, specific for each FLMA. 

Examples include the region and four letter 

Park/Area code for the NPS, state and field 

office for the BLM, or region and forest for 

the US Forest Service. The “Other” category 

should be used when the transportation asset 

does not occur on land managed by NPS, 

BLM, or USFS. Select the State and County 

for the corresponding dropdowns in the 

“Other” category. These are standardized on 

the electronic forms.

Hazard Type

The section is divided into two unstable 

slope hazard types, rockfall and landslides. 

Rockfall failure types include classic failure 

mechanisms (planar, wedge, and toppling),

raveling rock slopes (such as talus slopes)

(Hoek & Bray, 1981), rock avalanche, 

differential erosion (interlayered weak and 

stronger rock), and indeterminate rock 

failures. See Figure 1 for simplified 

schematic drawings of each rock failure 

type. Note that the ‘Indeterminate Rock 

Failure’ classification is primarily for sites 

where the rockfall mechanism is a complex 

interaction between multiple joints such that 

the straightforward planar, wedge, or 

toppling models are insufficient to describe 

the failure mechanism. This classification 

may be quite common in steep, hard, jointed 

rock cuts. Marking multiple selections is

permissible.

Figure 1. Simplified rock slope failure types 

(planar, wedge, and toppling adapted from 

Hoek & Bray, 1981).
60

Landslides can be generally classified by both 

their location relative to the route in question and 

by broad failure mode. Translational slides are 

typically composed of intact blocks that are 

moving on a flat or inclined discrete failure plane 

weaker than the surrounding geologic material. 

Rotational slides are typically formed by a 

circular failure surface, often on steep slopes. 

Debris flows are sudden, fast moving flows 

comprised of rock, soil, water and woody debris. 

Shallow slumps are common on transportation 

systems where the shoulder or outside lane are 

failing within the fill material or from debris 

above the road. Erosional failures are typical 

where the river system or culvert outfall is 

eroding the embankment or slope below the road 

which currently or may threaten the route in the 

future. See Figure 2 for simplified schematic 

drawings of each soil failure type, some 

modified from Cruden and Varnes, 1996 or 

Cornforth, 2005.

Road/Trail No. and Classification

Use the standard road numbering or naming 

approach for the FLMA’s road system. Note the 

road or trail’s classification (abbreviated as 

‘class’ on the form) according to the Agency’s 

road maintenance or use level 

classification/designation schema. 

Length of Affected Road/Trail

This is measured as the length of the road or trail 

adjacent to the hazard. For rockfall, it is 

measured from the start to the end of the cut 

slope or outcrop, not just where the highest level 

of rockfall activity is present. For landslides, it 

is measured from the beginning to the end of the 

slide where it is affecting the road or trail. For 

roadways, the affected length measurement 

includes both the paved road surface and the 

embankment on which the road is founded. For 

slides above the road, the affected length is 

measured as the distance over which the slide is 

Figure 2. Simplified landslide failure types 

(Cruden & Varnes, 1996; Cornforth, 

2005).
61

likely to impact the road or trail, from one end to the other. This measure is used in rating 

calculations.

Beginning and Ending Mile Markers

Use the posted mile markers and an offset with two decimal places. Ancillary vehicle devices 

(such as distance measuring devices) are helpful, but not required. Third-party mobile device 

odometer applications may also provide sufficiently accurate offsets. When markers are not 

posted, or used, a distance with direction from a main intersection may be sufficient to note in 

the comments section on the form. However, based on USMP practitioner feedback, it is 

strongly suggested that latitude and longitude locations be used as the primary locater because 

mile markers (when present) can change over time with realignments and administrative changes 

to road mileage. 

Side of the Road

Side of the road or trail is either left or right while travelling up mile point. When an unstable 

slope impacts both sides of the route, like a deep-seated landslide, note the upslope direction for 

side of the road or trail. Use cardinal directions when the route does not have mile markers 

posted.

Datum

Record the datum of the coordinate system. Note that WGS 84 is required for proper entry into 

the online mapping system so coordinates may need to be converted to WGS 84 if collected in 

another format.

Slope Height/Axial Length

This is measured as the maximum vertical 

height of the rock slope or the maximum axial 

length (slope distance) of a landslide feature. 

On short embankments where it appears that a 

fill failure is at fault, this measure is typically 

the axial length from the top to the base of fill; 

in other cases, engineering judgment may be 

needed. For debris flows, the axial length 

measurement could be in the thousands of feet 

due to the channel length (or axial length) being 

quite long. For safety and expediency, often 

times the axial length of the track of the debris 

flow can be measured through recent 

orthophotography or widely available electronic 

imaging available on the internet (see note 1 in 

Category K below). This measure is used in 

Figure 3. Examples of measuring vertical slope 

height on a rock slope and axial length on a 

landslide.
62

rating calculations. See Figure 3 for an example of evaluating slope height for rock slopes or 

landslides.

Slope angle

This is the average or representative angle of the rock cut slope/outcrop or the angle of the 

failing embankment or soil slope. This measure is not used in rating calculations.

Sight Distance

Sight distance is measured as the length of roadway from when a two-foot object is first seen 

from a driving position (3.5 feet from the road surface) until the object is reached. Sight distance 

is typically hindered by narrow shoulders, poor ditch vegetation control, and vertical and 

horizontal roadway curvature. The location’s sight distance should be measured in the lane 

direction with the worst visibility. This measure is used in rating calculations.

Roadway/Trail Width

This is measured as the available paved width of the roadway or trail, including paved shoulders 

as it exists at the time of rating. If the slope is rated while the road is partially closed due to 

debris or damage, repair of damage to the full width would require a new or updated rating to 

document the improvement to this unstable slope section. On unpaved routes, such as aggregate 

or native surface roads or trails, it would be the measured as the drivable or navigable width. 

Where width changes within a section, it should be taken at the narrowest part of the section. 

This measure is used in rating calculations.

Speed Limit

Record the speed limit in effect at the unstable slope. If lower advisory speeds are posted, those 

supersede the regular, posted speed limit. This measure is used in rating calculations.
63

Ditch Width/Depth/Foreslope

For rockfall areas, the roadside catchment 

ditch is an important rockfall risk reduction 

mitigation measure. A clean, wide ditch 

with a well-maintained foreslope is one of 

the most common risk reduction mitigation 

measures on transportation systems. Provide 

a measure of the range of ditch widths and 

depths and a representative foreslope angle. 

If needed, provide a note on the cleanliness 

of the ditch in the comments area of the 

form. This measure is not used in rating 

calculations.

Block Size/Volume

For rockfall only. Enter a reasonable value for the largest rock size (in feet) that could enter the 

roadway. This can be estimated by observation of rocks in the ditch, rocks that appear loose on 

the slope, or interviews with maintenance personnel (preferred). If a volumetric event is the 

dominant failure mechanism, enter the number of cubic yards that have or reasonably could enter 

the roadway. Again, interviewing maintenance personnel is very important to collect this 

historical information. This measure is used in rating calculations.

Annual Rainfall

Enter the average annual rainfall for the location. This measure is used in rating calculations. If 

a range is not available, then use the average rainfall for both ends of the minimum-maximum 

range. Precipitation maps are typically available on a statewide basis from Regional Climate 

Centers (such as the Western Regional Climate Center (http://www.wrcc.dri.edu/) or the state’s 

weather service. This measure is used in rating calculations.

Sole Access Route

If there are no alternative routes or detours in the event of a road closure, select “Yes”, otherwise 

select “No”. When seasonal closures create a sole access route condition, select “Yes”. This 

measure is not used in rating calculations.

Mitigation (Fixes) Present

If mitigation measures have been undertaken to halt or slow down a landslide, stabilize the rock 

slope, prevent rocks from reaching the roadway, etc., check the appropriate box. Typically, 

asphalt patches do not offer any appreciable stabilization effect on landslides and eventually can 

accelerate movement, so do not include patching or crack sealing as mitigation for landslides. 

On the paper field form, this field is listed as ‘Fixes Present’ due to space constraints. This 

Figure 4. Simple schematic of rock slope 

ditch.
64

measure is not used in rating calculations but provides a searchable field for uploading past 

unstable slope work documents and photos.

Photo/Documents Upload

Take (GPS-located) site photos and upload with your preferred hand-held device using the 

USMP Android or iOS application (recommended). Note that the size of photos and documents 

will be automatically reduced to 1600 by 1600 pixels to conserve space. Downloaded 

documents are limited to 10 Mb in size. For the paper forms, record the photo number range for 

the photos taken at the site, such as DSC005891.JPG to DSC005898.JPG, to help stay organized. 

These are often viewable on the digital camera’s image review screen.
65

PRELIMINARY RATINGS

The preliminary ratings are a select subset of the comprehensive rating categories. This subset is 

intended to provide guidance on whether to continue the rating assessment and can be used to 

include or exclude a candidate unstable slope location from the final database. If a site falls 

below the suggested score cutoffs, an agency may opt to completely exclude the site from the 

unstable slope database. If a site later becomes more of an issue, it should be assumed that the 

site has degraded from an acceptable to an unacceptable condition. If the site’s Preliminary 

Rating is above the suggested cutoff score of 21 (all scores of ‘3,’ with allowance of one ‘9’), the 

full field rating should be completed.

Note that all calculated category scores max out at 100 points.

When evaluating categories that are not calculated and have a max score of 81 points, it is not 

uncommon to determine that you are between two rating categories, for example, between 9 and 

27. When in doubt, it is advisable to select the higher rating value, and be consistent in your 

usage of this rule-of-thumb. When programmatic decisions are made for scoring cutoffs, you 

will want to make sure a seasoned geotechnical specialist will have an opportunity to review 

these unstable slopes that may be on the cusp of the cutoff score. As part of the conceptual 

design work, the geotechnical specialist will review the USMP rating and adjust it as needed. In 

some cases, this will lower some of the total scores below the cut line and they will not be further 

considered for conceptual design at this stage of the process. Selecting lower scores in some 

situations could result in missing some unstable slopes that deserved to be in the cutoff for 

further evaluation. This discussion will become clearer as you use the USMP process more and 

more.

Landslide-Specific Preliminary Hazard Ratings

Letters used in subsection headings correspond to specific fields in the slope rating forms (see 

Appendix B.)

A. Roadway Width Affected

When a part of the roadway or trail is lost, or blocked the following can occur: collision with the 

debris, driving off a scarp, attempting an evasive maneuver where the driver goes off the road or 

into oncoming traffic, or a hiker is forced into a hazardous situation that can lead to an accident. 

The hazard is related to the percentage of the roadway or trail affected.
66

Table 1. Preliminary Landslide Slope Rating – Roadway Impedance Category Narratives

3 points 0-5 percent The travel lanes are generally not affected by the landslide feature, but the 

available paved surface is reduced. A detour or traffic control (flagging) is typically not 

required except during maintenance activities. Trails typically are only slightly affected.

9 points 6-25 percent Events affect up to 25% of the travel lanes. Adequate paved surface is 

available to maneuver around the event. A detour is typically not required but traffic 

control may need to be utilized until the roadway is reestablished. A visual cue for 

tripping hazards may be needed on trails.

27 points 26-50 percent Events affect up to half of the surface dedicated to travel lanes. 

Maneuvering actions may still be possible by using paved or unpaved shoulders, if 

available. A detour or complete vehicle stoppage may be required. A visual cue for 

tripping hazards may be needed on trails and maintenance may be considered.

81 points 51-100 percent Events affect more than half of the road with limited paved surface 

available to maneuver around the event. A detour or stopping traffic flow is required. 

Trails may be closed and maintenance may be required to keep the trail open.

Example of roadways affected by landslide or settlement-related displacement are presented in 

Figures 5 and 6 below. As shown in Figure 5, the affected roadway width can vary throughout 

the site extents. The score should be based on the largest percentage observed at the site or 

reasonably predicted to occur in the event of landslide movement.

Figure 5. Embankment failure affecting up to 25 percent of the roadway. Forest Service 

Road 25, Milepost 30.
67

Figure 6. Embankment failure affecting entire roadway. Forest Service Road 25, Milepost 

25.

B. Slide/Erosion Effects

Unanticipated condition changes in a travel 

lane, such as those shown in Figure 7 at 

right, can result in unsafe maneuvers or loss 

of vehicle control. Larger obstructions 

increase the likelihood of an accident and 

require more maintenance effort and cost to 

repair. The category is scored by following 

the rating category narratives in the table 

below. Offsets indicated in the table below 

can be either horizontal or vertical.

Note: For trails, the slower driving speed 

indicated in the table below may be 

reflected in increased tripping hazards 

rather than lower speed limits.

Figure 7: Roadway deformation caused by 

embankment failure. Forest Service Road 90, 

Milepost 11.5.
68

Table 2. Preliminary Landslide Slope Rating – Slide/Erosion Effects Category Narratives

3 points Visible crack or slight deposit of material on road/minor erosion. For paved routes, slight 

pavement cracking or heaving, or a thin deposit of slide debris has occurred but they are small 

enough not to disturb traffic flow or require evasive maneuvers. Scheduled roadway maintenance 

is required. 

For thaw unstable slopes and/or unpaved routes, normal roadway speeds and driving behavior is 

maintained throughout the affected section. 

For trails, public activity is not affected.

9 points 1 inch offset, or 6-inch deposit of material on road/major erosion will affect travel in <5 years. For 

paved routes, a noticeable drop or heave in the pavement or a deposit of slide debris has occurred 

that requires lower speeds to traverse. Maintenance attention is required. 

For thaw unstable slopes and/or unpaved routes, a notable vertical movement is felt when 

traversing the affected roadway section at the speed limit.

For trails, public activity is not affected, but movement is noticeable.

27 points 2-inch offset or 12-inch deposit of material on road/moderate erosion impacting travel annually. 

For paved routes, a large drop or heave in the pavement or a deposit of slide debris has occurred 

that requires significantly lower speeds to traverse and may elicit unsafe driver reactions. 

Immediate maintenance attention is required.

For thaw unstable slopes and/or unpaved routes, breaking or evasive maneuvering is required when 

travelling the speed limit.

For trails, public activity is affected, prompting some to avoid the trail or turn back.

81 points 4-inch offset or 24-inch deposit of material on road/severe erosion impacting traffic consistently. A 

major drop or heave in the pavement or deposit of slide debris has occurred that cannot be 

traversed. Unsafe driver reactions are likely and immediate maintenance attention is required to 

reestablish safe traffic flow.

For thaw unstable slopes and/or unpaved routes, these sections have been marked by maintenance 

crews with warning signs, cones, or a temporary reduction of the speed limit.

For trails, the trail is periodically closed due to slide activity or the public needs to carefully 

traverse a severe offset.

C. Roadway Length Affected

The length of the roadway (or trail) affected by a landslide poses a hazard to the travelling public 

by increasing the likelihood of encountering the hazard, diverting into an adjacent lane, or 

increasing the distance or length of time the hazard will need to be avoided. To the agency, the 

length is proportional to the maintenance required and the costs associated with treatment. 

Longer slides will also require longer (both time duration and spatial length) lane closures during 

maintenance or repair activities. 

The length of roadway affected by landslide deformation is measured in the field, and the score 

is directly calculated from these field measurements, using the equation below. A graph of this 

equation is also provided for reference in Figure 8, as well as a table showing sample category 

scores, Table 3. 

SS = 3xx (max 100); = � h rr rrrr aa

25 ( )

Equation 1. Length of Roadway Affected Score
69

Figure 8. Chart illustrating the relationship between the length of roadway affected and the 

category score. The category score maxes out at an affected roadway length of about 440 

feet.

Table 3. Preliminary Landslide Slope Rating – Roadway Length Affected Sample 

Calculated Scores

3 points 25 feet

9 points 100 feet

27 points 225 feet

81 points 400 feet

Rockfall-Specific Preliminary Hazard Ratings

D. Ditch Effectiveness

The effectiveness of a ditch or catchment is measured by its ability to restrict falling rock from 

reaching the paved roadway, including any paved shoulder. The risk associated with a particular 

rock slope section is dependent on how well the ditch is performing in capturing rockfall. When 

little rock reaches the roadway, no matter how much rockfall is released from the slope, the 

danger to the public is low and the category score assessed is low. Conversely, if rockfall events 

are rare occurrences but the ditch is nonexistent, the resulting hazard is greater and a higher score 

is assigned to this category. Many factors must be considered in evaluating this category. The 

reliability of the result depends heavily on the rater's experience. Ditch Effectiveness is a 

subjective category. Figure 9 presents a graphic diagram of ditch effectiveness for guidance.

0

25

50

75

100

0 50 100 150 200 250 300 350 400 450

Roadway Length Category Score

Roadway Length Affected (ft.)
70

Figure 9. Ditch effectiveness explanatory diagram.

A wide fallout area does not necessarily guarantee that rockfall will be restricted from the 

highway. In estimating the ditch effectiveness, the rater should consider several factors, such as: 

1) slope height and angle; 2) ditch width, depth and shape; 3) anticipated rockfall event volume 

or rock size; and 4) impact of slope irregularities (launch features) on falling rocks. Evaluating 

the effect of slope irregularities is especially important because they can completely negate the 

benefits expected from a fallout area. Valuable information on past ditch performance can be 

obtained from maintenance personnel.

Table 4. Preliminary Rock Slope Rating – Ditch Effectiveness Category Narratives

3 points Good Catchment. All or nearly all falling rocks are retained in the catch ditch.

9 points Moderate Catchment. Falling rocks occasionally reach the roadway.

27 points Limited Catchment. Falling rocks frequently reach the roadway.

81 points No Catchment. No ditch, or ditch is totally ineffective. All or nearly all falling rocks 

reach the road.

E. Rockfall History

The rockfall history directly represents the known rockfall activity at the site. This information 

is an important check on the potential for future rockfalls. This information is best obtained 

from the maintenance personnel responsible for the slope. There may be no history available at 

newly constructed sites or where documentation practices are poor. The maintenance costs 

associated with a site may be the only information that reflects the rockfall activity. 
71

If the score a rater assigns to a section is determined not to correspond well with the rockfall 

history, a review of the rating is advisable.

Table 5. Preliminary Rock Slope Rating – Rockfall History Category Narratives

3 points Few Falls. Rockfalls occur only a few times a year (or less), or only during severe storms. 

This category is also used if no rockfall history data is available and evidence of rockfall 

is absent.

9 points Occasional Falls. Rockfall occurs regularly. Rockfall can be expected several times per 

year and during most storms.

27 points Many Falls. Typically, rockfall occurs frequently during a certain season, such as the 

winter or spring wet period, or the winter freeze/thaw, etc. This category is for sites where 

frequent rockfalls occur during a certain season but are not a significant problem during 

the rest of the year. This category may also be used where severe rockfall events have 

occurred.

81 points Constant Falls. Rockfalls occur frequently throughout the year. This category is also for 

sites where severe rockfall events are common.

F. Block Size or Volume per Event

Larger blocks or volumes of falling rock produce more total kinetic energy and greater impact 

force than smaller events. In addition, the larger events obstruct more of the roadway, reducing 

the possibility of safely avoiding the rock(s), and result in higher cleanup costs for the managing 

agency. In essence, the larger the blocks or event volume; the greater the hazard created; thus 

the higher the assigned score in this category.

This measurement should be representative of the type of rockfall event most likely to occur. As 

shown in Figure 10, debris currently contained in the roadside ditch can help generate a 

reasonable estimate. If individual blocks are typical of the rockfall at a site, as at the site in

Figure 10, then block size should be used for scoring. If a mass of blocks tends to be the 

dominant type of rockfall, volume per event should be used. A decision on which to use can be 

determined from the maintenance history, or estimated from observed conditions when no 

history is available. This measurement will also be beneficial in determining remedial measures.
72

Figure 10. Rockfall debris in the roadside ditch can be used to help assess both block 

size/event volume and failure type. Forest Service Rd 25, Milepost 25.

The category score is calculated according to the following equations. If the rater is uncertain, or 

both block size and volumetric events are present/feasible, rate the site using both equations and

record the higher of the two scores. If values for block size and volume are both are 

present/feasible and entered into the USMP electronic form under the block size/volume in Site 

Information, the larger of the two calculations will be recorded in this category. A pair of charts 

showing the exponential relationship between block size/event volume and category score is also 

presented in Figure 11 for reference, as are sample calculated category scores in Table 6.

BB SS SS SS = 3xx (max 100) ; = bb ( )

VV VV SS SS SS = 3xx (max 100) ; = �

3

3 �

Equation 2. Block Size and Volume Size Score
73

Figure 11. Chart pair illustrating the relationship between the block size and the category 

score and between the event volume and the category score. Note that the category score 

for block size maxes out for block sizes greater than 4 feet, while the category score for 

event volume maxes out for events over 12 cubic yards.

Table 6. Preliminary Rock Slope Rating – Block Size or Volume Size Sample Calculated 

Scores

Block Size Volume Size

3 points 1 foot 3 cubic yards

9 points 2 feet 6 cubic yards

27 points 3 feet 9 cubic yards

81 points 4 feet 12 cubic yards

Common Preliminary Slope Risk Ratings (Rockfall and Landslides)

G. Impact on Use 

Impacts on the transportation system due to a failure can be minimized if the expected impacts 

would be minimal, or if a detour around the site is available. The scoring should take into 

account a probable worst-case scenario, rockfall history, and geologic conditions when judging 

the impacts on traffic.

0

25

50

75

100

0 1 2 3 4 5

Block Size Category Score

Block Size (feet)

0

25

50

75

100

0 3 6 9 12 15

Event Volumne Category Score

Event Volume (cubic yds)
74

Table 7. Common Preliminary Rating – Traffic Impacts Category Narratives

3 points Full use continues with minor delay. A wide shoulder is available for traffic diversion for 

large slide events; small rockfall events contained in the ditch; nearby detours are 

available.

9 points Partial use remains. Use modification required, short (<3mile/30min.) detour available. 

Traffic control for a lane closure or detour is required for maintenance or clean-up. 

Detours are less than 3 miles or under 30 minutes in length for up to 1 day.

27 points Use is blocked – long (>30 min) detour available or less than 1 day closure for up to 1 

week. 

81 points Use is blocked – no detour available or closure longer than 1 week. Major reconstruction 

efforts with weeks or months closure with no detour.

H. Annual Average Daily Traffic (AADT) or Usage/Economic/Recreational Importance

This category is designed to capture route or trail importance and can be assessed using either 

quantitative or qualitative data. The AADT of a roadway provides a rough quantitative indicator 

of its impact on the regional economy and mobility of people, goods, and services. High traffic 

corridors will receive a higher risk score. The AADT score is based on the following equation:

SS = 3xx (max 100) ; = �AAAAAAAA

50

Equation 3. Annual Average Daily Traffic Score

Figure 12. Chart illustrating the relationship between AADT and the category score. The 

category score maxes out at an AADT of approximately 880 vehicles.

0

25

50

75

100

0 100 200 300 400 500 600 700 800 900

AADT Category Score

Annual Average Daily Traffic (AADT)
75

For those roads or trails where AADT information is not available (uncheck the “Use AADT in 

Calculation” box on the electronic form), a qualitative score relating to usage and relative 

economic/recreational importance is applied, as shown in Table 8. 

Table 8. Common Preliminary Rating – AADT Sample Calculated Scores

AADT Score* Qualitative Usage/Economic/Recreational Importance Score*

3 points 50 Rarely used. Insignificant economic and/or recreational importance

9 points 200 Occasionally used. Minor economic and/or recreational importance

27 points 450 Frequently used. Moderate economic and/or recreational importance

81 points 800 Constantly used. Significant economic and/or recreational importance

*The highest rating of the two category narratives is applied

For a site where both quantitative (AADT) and qualitative (i.e., relative importance) data is 

available, both categories should be evaluated, and the highest resulting score given to the site. 

For example, a roadway that is the only route to a popular trail may be of moderate recreational 

importance (27 pts), but the AADT is only 200 (9 pts). For this site, the higher score of 27 

would be used.

Preliminary Rating Score

Following the completion of the Preliminary Ratings, the slopes can be categorized into one of 

three categories of ‘Good’, ‘Fair’, or ‘Poor.’ This language is consistent with Federal legislation 

for bridge and pavement asset management regulations. Depending on your agency’s approach 

to conducting ratings, ‘Good’ slopes may not require further assessment. If a slope is 

categorized as ‘Fair’ or ‘Poor,’ this may trigger further evaluation and completion of the Hazard 

and Risk Categories. At a minimum, Fair and Poor sites should have the full detailed ratings 

performed.

Table 9. Good, Fair, and Poor Score Ranges.

Descriptor Preliminary Score Range

Good 15-21

Fair 22-161

Poor >161
76

DETAILED HAZARD RATINGS

The Detailed Ratings complete the suite of possible rating categories. Depending on an agency’s 

rating approach, these categories will be evaluated if the Preliminary Rating is above a cutoff 

score, proposed as 21 in this manual, but official policy or documented decisions regarding the 

rational for adjusting these cutoff scores between good, fair, and poor slopes are left to the 

discretion of each participating agency or department.

Common Hazard Ratings (Rockfall and Landslides)

I. Slope Drainage

In conjunction with rainfall quantity, the ability of the slope materials to be free draining and the 

presence of seeps and/or springs (indicating a relatively constant water source) provides 

information on the ability of the slope to cope with rainfall and freeze-thaw events. This 

subcategory is based on subjective evaluations. Note that rating this category at different times 

of the year may produce different results as creeks and springs may dry up during late summer 

months. For guidance in field evaluations, Figure 13 is provided below, and rating category 

narratives are provided in Table 10.

Figure 13. Guidance figure for evaluating slope drainage.
77

Table 10. Common Hazard Rating - Slope Drainage Category Narratives

3 points Well Drained. Slope appears dry or well drained; surface runoff well controlled; slope is 

dry hours after rain events; or a functioning drainage system is installed.

9 points Moderately Well Drained. Water is intermittently on slope; moderately well drained; 

surface runoff moderately controlled; slope is dry days after rain events.

27 points Moderately Poorly Drained. Water usually on slope; poorly drained; surface runoff 

poorly controlled; slope is still wet a week or two following rain events, but may dry 

during prolonged dry spells.

81 points Poorly Drained. Water always on slope; very poorly drained; or surface water runoff 

control not present.

J. Annual Rainfall

In conjunction with slope drainage, the amount of annual rainfall at a site is a rough indicator of 

the frequency and potential for high pore-water pressures to accumulate. Areas with frequent, 

intense storms typically have more unstable rock and soil slopes. 

This subcategory is rated based on rainfall ranges. A rock slope in an area with 12 inches of 

average annual rainfall and a rock slope in an area with 29 inches of average annual rainfall 

should both receive a score of 9 points in this category. Because annual rainfall cannot be 

estimated during a site visit, annual rainfall data must be obtained from an appropriate regional 

or local source before starting field work.

Table 11. Common Hazard Rating – Annual Rainfall

3 points 0-10 inches of rain annually

9 points 10-30 inches of rain annually

27 points 30-60 inches of rain annually

81 points 60+ inches of rain annually
78

K. Slope Height or Axial Length of Slide

This category evaluates the risk 

associated with the height of a rock slope 

or axial length of a landslide or debris 

flow. The slope height measurement is to 

the highest point from which rockfall is 

expected or the axial length (slope 

distance) of a landslide, as shown in the 

adjacent figure. The Site Information 

portion of the form should already 

contain these measurements.

If rockfall is generated from the natural 

slope above the cut slope, the slope height 

measurement should include both the cut 

height and the additional vertical height 

on the natural slope to the rockfall source. 

On a landslide, the distance from scarp to 

toe should be measured. For debris flows 

the approximate axial or channel distance 

from the roadway to the source area 

should be entered.1

In cold climates, thaw instability can affect roadway embankments that run over relatively flat 

ground. In those cases, the axial length of the slide is assumed to be equal to the axial length of 

the embankment fill prism. Although thaw instability can affect an embankment over many 

hundreds of feet, which is captured in the Roadway Length Affected in Category C, the 

maximum slump or settlement caused by thawing soils cannot exceed the height of the roadway 

embankment.

This category is directly measured and scored using the equation presented below. A chart 

relating slope height/axial length and category score is presented for reference, as is a table 

containing sample calculated category scores.

SS = 3xx (max 100) ; = ss h h aa aa h

25

Equation 4: Slope Height or Axial Length Score

 

1 Note: Channel length measurements for debris flows do not need to be precise, as the rating category score maxes 

out at a slope height/axial length of approximately 105 ft. An estimated channel length of 1,000 feet or of one mile 

has the same net effect: maxing out the rating score for this category. 

Figure 14. Examples of measuring vertical slope 

height on a rock slope and axial length on a 

landslide.
79

Figure 15. Chart illustrating the relationship between the slope height or axial length and 

the category score. The category score maxes out at height/axial length of approximately 

105 feet

Table 12. Common Hazard Rating - Slope Height or Axial Length of Slide Sample 

Calculated Scores

3 points 25 feet

9 points 50 feet

27 points 75 feet

81 points 100 feet

Landslide-Specific Hazards 

L. Thaw Stability (Cold Climates)

Roads and embankments founded on melting permafrost become unstable, creating a rough and 

wavy driving surface along with other roadway hazards. Melting slopes above the road become 

unstable and have the potential to impact the roadway. Depending on the gradation, soils 

containing frozen water pose maintenance problems if the ice thaws. The magnitude and 

likelihood of related problems are higher for finer-grained soils that contain large amounts of ice 

or ice layers. Where the ice-bearing layers are not visible, base the thaw stability on the relative 

performance of the roadway.

While performing field ratings, the subsurface condition described below can be estimated by the 

surficial expressions of thaw instability. For instance, thaw instability is often expressed by 

wavy pavements. Low amplitude ‘waves’ that developed over a long period of time represent 

greater thaw stability than higher amplitudes that develop quickly or need frequent repair. 

0

25

50

75

100

0 10 20 30 40 50 60 70 80 90 100 110

Height/Axial Length Category Score

Slope Height or Axial Length (ft)
80

Examination of nearby outcrops or cuts may expose ice conditions. A review of local geology 

and geomorphic features indicating ice (patterned ground, palsas, pingos, etc.) assists with 

interpreting subsurface ice conditions.

Table 13. Detailed Landslide Slope Rating – Thaw Stability Category Narratives

3 points Unfrozen / Thaw Stable. Soil may be coarse- or fine-grained. No ice is visible with the 

naked eye, but if present, it does not occupy space in excess of the original voids. These 

soils are usually thaw-stable. No thaw unstable slopes should be rated in this category

9 points Slightly Thaw Unstable. Soil is coarse-grained. Ice occupies space equal to, or in excess 

of, the original voids. It is present as crystals or lenses visible with the naked eye. These 

soils may be thaw-unstable depending on soil density. Few thaw unstable slopes should be 

rated in this subcategory.

27 points Moderately Thaw Unstable. Soil is fine-grained. Ice occupies space equal to, or in excess 

of the original voids and is present as crystals or lenses visible with the naked eye. These 

soils are typically thaw-unstable. Most thaw unstable slopes are rated in this category 

based on relative performance of the roadway.

81 points Highly Thaw Unstable. Soil layers contain significant quantities of ice well in excess of 

the original void space. The ice is readily visible with the naked eye and is present as 

large lenses or as separate ice layers. These materials are highly thaw-unstable. Any 

embankment sections with characteristics indicating a likelihood or history of rapid failure 

or severe displacement due to the presence of thaw unstable materials should be rated in 

this subcategory.

M. Instability-Related Maintenance Frequency

As instability-related movement progresses, 

trail or roadway deformation begins to hamper 

performance, and maintenance attention is 

required to ensure that the site remains 

passable. Slide maintenance puts staff and 

equipment in, or near the road, which may 

impede traffic and exposes both maintenance 

personnel and the general public to potential 

hazards. The more often maintenance activity 

is required, the greater the hazards posed to the 

public and maintenance staff, and the greater 

the maintenance cost.

Maintenance frequency should be determined 

through an interview with maintenance 

personnel, but it can be initially estimated by 

the rater based on field observation, and refined in interviews with maintenance personnel

following field work. This category is rated subjectively based on the rating category narratives 

in Table 14 below.

Figure 16. Instability requiring repeated 

maintenance attention. Forest Service Road 

25, Milepost 25.
81

Table 14. Detailed Landslide Slope Rating – Maintenance Frequency Category Narratives

3 points Every 10 years. Events requiring maintenance intervention are relatively rare or 

nonrecurring and/or the repair activities can typically be completed using standard 

equipment with minimal impacts to traffic flow.

9 points Every 5 years. Maintenance intervention is required occasionally and/or the repair 

activities can usually be completed in less than a day using standard equipment, but traffic 

flow is reduced and flagging is required.

27 points Every 2 years. Maintenance action is routinely required and/or the repair activities require 

non-standard equipment or more than one day to complete; or the traffic flow is 

significantly impeded for more than a day and flagging is required.

81 points Every year. Maintenance is required one or more times per year or wherever major events 

have occurred requiring several days to restore traffic. This category also applies if an 

outside contractor is required.

N. Movement History

The rate of slide movement per event and the frequency of events relate to public hazard and 

maintenance requirements. Higher rates of movement are more likely to create unanticipated 

roadway conditions that require immediate, unscheduled maintenance. This category should be 

rated based on input from maintenance personnel, since it is difficult to accurately assess an 

annual rate of movement from a single site visit. Movement magnitudes focus on paved 

roadways. For trails and unpaved roads, evaluate the impacts of movement according to the 

descriptions in Table 15.

Table 15. Detailed Landslide Slope Rating – Movement History Category Narratives

3 points Minor movement or sporadic creep. The rate of movement is low and non-continuous. 

Pavement disturbance is minor on an annual basis and maintenance requirements are 

minimal and carried out as a scheduled activity. 

9 points Up to 1 inch annually or steady annual creep. The rate of movement is low but 

continuous. Corridor maintenance is routinely required to avoid closures but maintenance 

action can generally be conducted on a scheduled basis.

27 points Up to 3 inches per event, one event per year. The rate of movement is moderately high. 

Events occurring more than twice a year that require immediate and unscheduled 

maintenance are a persistent maintenance problem.

81 points >3 inches per event, >6 inches annually, or more than 1 event per year (includes all 

debris flows). The rate of movement is high with significant travel disturbance 

developing quickly. Aggressive, unscheduled maintenance intervention is required to 

maintain traffic flow and correct unsafe conditions.

Rockfall-Specific Hazards

O. Rockfall Related Maintenance Frequency

The required frequency of maintenance is an indicator of both rockfall activity and long-term 

cost to the agency. When there is little to no maintenance required and only scheduled ditch 
82

cleaning required, both maintenance staff and the travelling public are typically not exposed to 

risk and little cost to the agency is required. As rockfall activity increases at a site, additional 

surveillance activities specifically checking for rockfall activity may be warranted after storm 

events and rockfall clean-up activities increase. An example of a rock slope requiring almost

daily maintenance attention is shown in the figure below.

Figure 17. Constant rockfalls occur at this rock slope on the Glenn Highway in Alaska. 

Maintenance personnel stockpile the daily debris in this pullout for regular removal. This 

rock cut received a category score of 81.

Maintenance frequency should be determined through an interview with maintenance personnel, 

but it can be initially estimated from conditions observed at the site, as in the figure above, but 

category ratings should be confirmed through discussions with maintenance personnel following 

field work.

Table 16. Detailed Rock Slope Rating – Rockfall-Related Maintenance Frequency Category 

Narratives

3 points Normal, scheduled ditch maintenance. Only routine, scheduled ditch maintenance is 

required on an infrequent (3-5 year) basis. Few, if any rocks accumulate in ditch between 

maintenance intervals.

9 points Road Patrols conducted after storm events. Maintenance staff only actively search for

rock within the ditch or roadway after extreme storm events. Ditch cleanout of rock 

debris is infrequently required beyond scheduled ditch cleaning.

27 points Routine seasonal road patrols. Maintenance staff routinely patrol for rock during typically 

high rockfall seasons (fall, winter, spring). Ditch cleanout of rock debris is occasionally 

required beyond scheduled ditch cleaning.

81 points Year-round road patrols. Maintenance staff routinely patrol for rock year round. Ditch 

cleanout of rock debris is frequently required beyond scheduled ditch cleaning.
83

Geologic Character

The geologic conditions of the rockfall section are evaluated with these categories. Since the 

conditions that cause rockfall generally fit into two categories, Case 1 and Case 2 rating criteria 

have been developed. Case 1 is for slopes where joints, bedding planes, or other discontinuities 

are the dominant structural features that lead to rockfall. Case 2 is for slopes where differential 

erosion or oversteepening is the dominant condition that controls rockfall. 

Raters should use the case that best fits the slope for the rating. If both situations are present, and 

it is unclear which dominates, both can be scored, but only the worst case (highest score) is used 

in the rating. 

Case 1

Rockfall from Case One slopes occurs as a 

result of movement along discontinuities. 

The word “joint” as applied here, which 

represents all possible types of 

discontinuities, including bedding planes, 

foliations, fractures, and faults. The term 

“continuous” refers to joints that are greater 

than 10 feet in length. The term “adverse” 

applies not only to the joint's spatial 

relationship to the slope, but also to such 

things as rock friction angle, joint infilling, 

and the effects of water on slope stability, if 

present. An example of a rock slope in 

geologic Case 1 is shown in Figure 18.

P. Case 1 - Structural Condition

Jointed rock is typically more prone to rockfall than massive rock. Movement occurs along these 

joints where the resistance to movement is significantly less than the intact strength of the rock 

itself. When the joints are orientated adversely to the slope, the potential for rockfall is greater. 

Adverse joints are those that singularly, or in combination with other joints, make planar, 

circular, block, wedge or toppling failures kinematically possible.

Figure 18. Rock cut where failure is 

controlled by interaction between geologic 

structure and rock friction. Forest Service 

Road 90, Milepost 13.
84

Table 17. Detailed Rock Slope Rating – Case 1 Structural Condition Category Narratives

3 points Joints with favorable orientations. Slope contains jointed rock with no or very few 

adversely oriented joints.

9 points Random (both favorable and unfavorable) orientations. Slope contains randomly oriented 

joints creating a variable pattern. The slope is likely to have some scattered blocks with 

adversely oriented joints, but no dominant adverse pattern is present.

27 points Discontinuous joints with adverse orientations. Rock slope exhibits a prominent joint 

pattern with an adverse orientation. These features have less than 10 feet of continuous 

length.

81 points Continuous joints with adverse orientations. Rock slope exhibits a dominant joint pattern 

with an adverse orientation and a length greater than 10 feet.

Q. Case 1 - Rock Friction

The potential for rockfall caused by movement along discontinuities is controlled by the 

condition of the joints. The condition of the joints is described in terms of micro and macro 

roughness. The Joint Roughness Coefficient (JRC) (Barton & Choubey, 1977) can be used as a

guide for approximating roughness.

This parameter directly affects the potential for a block to move relative to another. Friction 

along a joint, bedding plane, or other discontinuity is governed by the macro and micro 

roughness of the surfaces. Macro roughness is the degree of undulation of the joint relative to 

the direction of possible movement. Micro roughness is the texture of the surface. Rockfall 

potential is greater on slopes where the joints contain hydrothermally altered or weathered 

material, movement has occurred causing slickensides or fault gouge to form, or the joints are 

open or filled with water. 

Table 18. Detailed Rock Slope Rating – Case 1 Rock Friction Category Narratives

3 points Rough. Irregular. The surface of the joints is rough and the joint planes are irregular 

enough to cause interlocking.

9 points Undulating. Rough but without the interlocking ability, judged on the macro scale.

27 points Planar. Macro smooth and micro rough joint surfaces. Friction is derived strictly from the 

roughness of the rock surface.

81 points Clay Infilling, Open, or Slickensides. Low friction materials separate the rock surfaces, 

negating any micro or macro roughness of the joint surfaces. Slickensided joints also 

have a lower friction angle because joint surfaces have been smoothed by movement, 

whether related to slope stability or tectonic movement.
85

Figure 19. Joint Roughness Coefficient (JRC) guide, modified here as dimensionless. Low 

JRC values for macro roughness (large planar joints) joints would equate to a higher score. 

Rough, irregular joints (high JRC), both in macro and micro scale, score low.

27

.

.

.

.

.

.

.

9

.

.

.

.

.

.

.

3 

Approximate 

rating progression 

using the JRC as a 

guide.
86

Case 2

This case is used for slopes where differential 

erosion or oversteepening is the dominant 

condition that leads to rockfall. Erosion features 

include oversteepened slopes, unsupported rock 

units (overhangs), or exposed resistant rocks on a 

slope, which may eventually lead to a rockfall 

event. An example of this geologic case is shown 

in Figure 20.

R. Case 2 - Structural Condition

Rockfall is commonly caused by erosion that 

leads to a loss of support, either locally or 

throughout a slope. The types of slopes that may 

be susceptible to this condition are: layered 

geologic units containing more easily erodible 

units that undermine more durable rock; talus 

slopes; highly variable geologic units, such as 

conglomerates, and mudflows, that weather 

differentially, allowing resistant rocks and blocks 

to fail, and rock/soil slopes that weather allowing 

rocks to fall as the soil matrix material is eroded.

Table 19. Detailed Rock Slope Rating – Case 2 Structural Category Narratives

3 points Few Differential Erosion Features. Minor differential erosion features that are not 

distributed throughout the slope.

9 points Occasional Differential Erosion Features. Minor differential erosion features that are 

widely distributed throughout the slope.

27 points Many Differential Erosion Features. Differential erosion features that are large and 

numerous throughout the slope.

81 points Major Differential Erosion Features. Severe cases, such as dangerous erosion-created 

overhangs, or significantly oversteepened soil/rock slopes or talus slopes.

S. Case 2 - Differential Erosion Rate

The materials comprised in a slope can have markedly different characteristics that control how 

rapidly weathering and erosion occur. As erosion progresses, resulting in portions of the slope 

becoming unsupported, the likelihood of a rockfall event increases. 

Figure 20. Rock cut where differential erosion 

is the dominant cause of failure. Forest Service 

Road 25, Milepost 30.
87

The rate of erosion on a Case 2 slope directly relates to the potential for a future rockfall event. 

As erosion progresses, unsupported or oversteepened slope conditions develop. The impact of 

the common physical and chemical erosion processes, as well as the effects of human actions, 

should be considered. The degree of hazard caused by erosion and thus the score given this 

category, should reflect the rate at which erosion is occurring; the size of rocks, blocks, or units 

being exposed; the frequency of rockfall events; and the amount of material released during an 

event.

Table 20. Detailed Rock Slope Rating – Case 2 Differential Erosion Rate Category 

Narratives

3 points Small Difference. Erosion features take many years to develop. Slopes that are near 

equilibrium with their environment are covered by this category.

9 points Moderate Difference. The difference in erosion rates allows erosion features to develop 

over a period of a few years.

27 points Large Difference. The difference in erosion rates allows noticeable changes in the slope 

to develop annually.

81 points Extreme Difference. The difference in erosion rates allows rapid and continuous 

development of erosion features.
88

DETAILED RISK RATINGS (ROCKFALL AND LANDSLIDES)

V. Route Width or Trail Width

The roadway or trail width is measured perpendicular to the centerline. This category measures 

the available maneuvering width of the road or trail, and captures the ability of a traveler to 

navigate around unforeseen roadway or trail hazards. For example, if a traveler notices rocks in 

the road, or rocks falling, it is possible for the driver or hiker to react and take evasive action to 

avoid them. The more room there is for this maneuver on a roadway, the greater the likelihood 

the driver will successfully miss the rock without hitting some other roadside hazard or 

oncoming vehicle. For a trail, greater room for maneuvering reduces the likelihood that a user 

will trip or be compelled to exit the trail in order to avoid the obstacle.

Roadway width is measured as the available paved width of the roadway or trail, including 

paved shoulders as it exists at the time of rating. If paved, the edges of pavement define the 

roadway. It is difficult to get uniform estimates among different raters about what is unpaved 

shoulder and what is an unmaneuverable side slope. For that reason, the unpaved shoulders are 

not included in the measurement. On unpaved routes, such as aggregate or native surface roads 

or trails, it would be the measured as the drivable or navigable width. Where width changes 

within a section, it should be taken at the narrowest part of the section. On divided roadways, 

only the portion of the roadway available to the driver for maneuvering should be measured. If 

the slope is rated while the road is partially closed due to debris or damage, repair of damage to 

the full width would require a new or updated rating to document the improvement to this 

unstable slope section. 

This category score is based on direct measurements according to the equations below. Graphs 

of the category scores for roads and for trails are also provided for reference in 

Figure , and sample calculation results are provided in Table .

SS = 3xx (max 100); where

= 44 − RR h ( )

8 for vehicles, = 18 − h ( )

4 for trail traffic

Equation 5. Roadway Width Score
89

Figure 21. Chart pair illustrating the relationship between the paved roadway with and the 

category score and between the trail width and the category score. Note that the category 

score for paved roadway width maxes out at a width of 10 feet, while the category score for 

trail width maxes out at a width of 1 foot.

Table 21. Risk Rating – Roadway Width Sample Calculated Scores

Roadway Width Trail Width

3 points 36 feet 14 feet

9 points 28 feet 10 feet

27 points 20 feet 6 feet

81 points 12 feet 2 feet

W. Human Exposure Factor

The Human Exposure Factor evaluates the potential for a roadway or trail user to be involved in 

an unstable slope event. This risk is associated with the percentage of time a route user is 

present in the evaluated section. The percentage is obtained by using the formula based on slope 

length, average annual daily traffic (AADT), and the posted or advisory speed limit (or average 

walking speed, 2.7 mph(Knoblauch, et al., 1996) at the site. If a different walking speed is 

entered, note study and/or rationale in the comment field.

A rating of 100% means that, on average, a user will be within the defined unstable section 

100% of the time. Where high AADTs or longer slope lengths exist, calculated values can be 

greater than 100%, meaning that at any particular time, more than one user is present within the 

measured section. The result also reflects the significance of the route.

This category is scored using direct measurements following the equation below.

0

25

50

75

100

10 20 30 40 50

Route Width Category Score

Paved Roadway Width (feet)

0

25

50

75

100

0 5 10 15 20

Trail Width Category Score

Trail Width (feet)
90

SS = 3xx (max 100) ; =

�

AAAAAAAA

24 × ss h ( ) ×100

ss ss �

12.5

Equation 6. Human Exposure Factor Score

Table 22. Risk Rating – Human Exposure Factor Sample Calculated Scores

3 points Human Exposure 12.5% of the time

9 points Human Exposure 25% of the time

27 points Human Exposure 37.5% of the time

81 points Human Exposure 50% of the time

X. Percent Decision Sight Distance (PDSD) or Avoidance Ability on Trails

The Percent Decision Sight Distance (PDSD) category compares the amount of sight distance 

available through an unstable slope section to the optimal sight distance for a 

speed/path/direction change. 

For roadways, sight distance is measured as the length of roadway from when a two-foot object 

is first seen from a driving position (3.5 feet above the road surface) until the object is reached. 

Decision sight distance (DSD) is the length of roadway, in feet, required by a driver to perceive a 

problem and then bring a vehicle to a stop. The required DSD increases with increased vehicle 

speed and this distance is critical when obstacles in the road surface are difficult to see, or when 

unexpected or unusual maneuvers are required. Decision sight distances prescribed by 

AASHTO for rural roads for typical posted speeds are presented in Table 23 below.

Table 23. AASHTO Recommended Minimum Decision Sight Distance for selected speed 

limits

Posted Speed 

Limit (mph)

AASHTO Recommended 

Minimum Decision Sight Distance 

(ft)

25 375

30 450

35 525

40 600

45 675

50 750

55 875

60 1,000

65 1,050
91

The DSD is critical when obstacles on the road are difficult to see, or when unexpected or 

unusual maneuvers are required. Throughout an unstable slope section, the sight distance can 

change appreciably. Horizontal and vertical highway curves along with obstructions such as 

rock outcrops and vegetation can severely limit a driver’s ability to notice and react to a rock in 

the road. In calculating this category score, the sight distance is determined in both travel 

directions, and the most restricted sight distance should be used. Both horizontal and vertical 

sight distances are evaluated.

The measurement, generally made with a roller tape or laser range finder, is the distance required 

for a two-foot object positioned on the fog line (or on the edge of pavement if there is no fog

line) to disappear from view at an eye height of 3.5 feet above the road surface. The posted, or 

advisory speed limit throughout the rockfall section is used because unstable slopes are often 

located within highway curves, where the posted advisory speed limit is lower than the highway 

design speed. Formally, AASHTO standards require placing the object in the travel lanes and 

measuring it from a driver’s position near the centerline; however, ratings are typically 

performed under live traffic and this would be an unsafe practice for both the rater and the 

public. 

For roadways, this category is scored based on direct measurements using the equation below.

SS = 3xx; =

120 − � SS h

AAAA AAAAAA RR SS h × 100�

20

Equation 7. Percent Decision Sight Distance Score

Avoidance ability on trails should be estimated qualitatively based on observations at the time of 

rating. Since comparable design standards for trails do not exist, the rater should judge the 

ability of a hiker to avoid a sudden rockfall, broken down into easily, moderately, difficult, or 

very difficult to avoid. For example, a hiker traversing a trail through a flat grassy area would 

easily avoid sudden hazards by leaving the trail into predictable surroundings; on a trail next to a 

shallow stream without a drop off it may be moderately difficult to avoid a hazard; on a 

boardwalk trail with handrails and a five foot drop to wetlands below it may be difficult to avoid 

a sudden hazard; and on a narrow trail with a tall cliff below and loose rocks it may be very 

difficult to avoid a sudden hazard. Below, Table 24 provides the qualitative descriptions and 

scoring breakdowns for trails, and the calculated value breakdown for roadways based on 

Equation 7 above. 

Table 24. Risk Rating – Percent Decision Sight Distance Sample Calculated Scores

Roadways Trails

3 points Adequate, 100% of design value Hazards easily avoided

9 points Moderate, 80% of design value Hazards moderately difficult to avoid

27 points Limited, 60% of design value Hazards difficult to avoid

81 points Very Limited, 40% of design value Hazards very difficult to avoid
92

Y. Right of Way Impacts if Left Unattended

Adjacent land owners may be impacted by unstable slopes retrogressing to property boundaries 

and beyond. If structures or other transportation systems are potentially impacted by unstable 

slopes, then the risk to the agency increases. Maps displaying right of way (ROW) are helpful 

when performing evaluations. 

To offer additional flexibility in this rating category for other agency or department facility 

assets beside roads and trails, raters may opt to use this category to evaluate the impact of 

landslides or rockfall on their built infrastructure. For instance, where no facilities are in harm’s 

way, 3 points would be scored. Minor, easily replaceable facilities, such as benches or railings 

could be 9 points. Retaining walls, bear boxes, or drinking fountains could be 27 points, and 81 

points could be assigned to significant structures such as ranger stations, restrooms, or other 

occupied structures.

Table 25. Risk Rating – Right of Way Impacts Category Narratives

3 points No ROW implications. Unstable slopes very unlikely to extend beyond agency ROW. 

9 points Minor effects beyond ROW. Retrogressing unstable slopes impacting non-agency ROW, 

but adjoining landowner indifferent to minor impacts. Minor impacts include overburden 

slumping, minor drainage changes, or rock slope crest retrogression.

27 points Private property, no structures affected. Unstable slopes actively retrogressing into 

private property but not impacting or likely to threaten structures. ROW acquisition of 

private lands may be a remote option.

81 points Structures, roads, RR, utilities, or parks affected. Retrogressing unstable slopes actively 

threatening adjacent structures, transportation systems, or Federal or State Park lands. In 

this score range, ROW acquisition of private lands may be a viable option. Coordination 

of mitigation approaches with outside agency landowner(s) will likely be required.

Z. Environmental/Cultural Impacts if Left Unattended

If the unstable slope is left unattended, impacts to the environment or cultural resources may 

occur. These impacts can include siltation of streams, culvert plugging, subsequent fish passage 

blocking, habitat impacts, or damage to historic features or sites. Due to the highly variable 

nature of potential environmental impacts, a range of environmental and cultural descriptions are 

used for this category. The rater should select the category containing the highest environmental

or cultural impact if left unattended. If these impacts are anticipated, a review by environmental 

and/or cultural resource professionals may be recommended. 

Preservation of paleontological resources, which is mandated by law, should also be considered

in this category and separated similarly to the rating categories provided for environmental and 

cultural impacts presented in Table 26.
93

Table 26. Risk Rating – Environmental/Cultural Impacts Category Narratives

3 points None/No potential to cause effects. No known sensitive environmental issues are present 

or anticipated if a probable worst-case scenario occurs. Hazard does not have the potential 

to cause effects on historic properties, assuming such historic properties are present (36 

CFR 800.3(a)(1)).

9 points Likely to affect/No historical property affected. If a probable or historically common 

failure occurs or the slope retrogresses, minor environmental impacts are anticipated, but 

adverse impacts are not anticipated. Historic properties are present but the hazard will 

have no effect upon them (36 CFR 800.4(d)(1))

27 points Likely to Adversely Affect/Finding of No Adverse Effect. If a probable or historically 

common failure occurs or the slope retrogresses adverse impacts are anticipated. Historic 

properties are present but the hazard will require modification or conditions imposed 

should the hazard continue untreated (36 CFR 800.5(a)(2)(vii)(b)).

81 points Current adverse effects/Adverse Effect. Current conditions are causing adverse 

environmental effects. An adverse effect is found when an undertaking may alter, directly 

or indirectly, any of the characteristics of a historic property (36 CFR 800.5(a)(1)).

AA. Maintenance Complexity

Complexity of maintenance following routine and/or a probable worst case scenario rockfall or 

landslide event is indicative of the maintenance costs and associated hazards. Maintenance could 

be as simple as cleaning rocks off the road during routine road patrols or as complex as the 

maintenance of unstable slope remediation systems, such as rockfall attenuator fences or 

construction of mechanically stabilized earth (MSE) walls to remediate an embankment 

landslide. This information should be gathered during the interview of maintenance personnel, 

specifically what the personnel capabilities and equipment is available to the agency to perform 

maintenance activities in the rating corridors.

Table 27. Risk Rating – Maintenance Complexity Category Narratives

3 points Routine effort/in-house. Maintenance staff typically deal with unstable slopes with roadgoing equipment such as a pickup with a blade, particularly effective with rockfall 

incidences. Trails require typical maintenance activities.

9 points In-house maintenance/special project. Maintenance of the site requires mobilization of 

specialized equipment such as a backhoe, excavator, paver, or guardrail post driver. Trails 

may require blasting and/or geotextile fabrics with existing surface materials to maintain 

the trail route and width. 

27 points Specialized equipment/contract. Maintenance requires specialized equipment to be 

mobilized a significant distance or requires assistance from an outside contractor. More 

involved maintenance may require basic engineering efforts (such as subgrade design or 

asphalt mixes). This would be similar for trails, such as renting a trail excavator and small 

dump to haul materials and may require basic engineering efforts (such as short retaining

walls or realignments of trail sections).

81 points Complex or dangerous effort/location/contract. Specialty contractor is required to 

perform maintenance activities (such as maintaining rockfall attenuator fences); more 

complex maintenance designs (such as subgrade reinforcement, tall MSE walls, or 

rockfall mitigation) requiring geotechnical design efforts; or difficult/dangerous access 

(rope access, spider hoe) is required.
94

BB. Event Cost

The estimated, or actual cost if available, to maintain or repair a probable worst-case scenario or 

a historically bad failure should be considered. The costs should be considered at comparable 

private-sector equipment rental and operator rates. If an extreme event requires outside 

assistance (planning, design, and/or construction) the costs should include both those outside 

costs and the agency contracting and supervisory costs. These rating categories are based on 

typical spending and contractual authorities in Federal agencies and departments. Again, 

interviewing maintenance personnel to determine the previous expenditures for similar unstable 

slope work proposed can be very helpful when teamed with a brief field review of the unstable 

slope sites to determine the likely, and appropriate rating for this category. 

Table 28. Risk Rating – Event Costs Category Narratives

3 points $0-2k. Maintenance efforts and costs involve only agency maintenance staff using 

existing equipment. No design work required.

9 points $2-25k. Event cost and response is more involved and may include input from agency 

engineering staff. 

27 points $25-100k. Costs indicate extensive, multi-day efforts and likely input from engineering 

staff. Costs may include outside contractors and engineering costs.

81 points $ >100k. Large contract with significant outside contractor and engineering costs.
95

REFERENCES FOR APPENDIX A

Barton, N. & Choubey, V., 1977. The Shear Strength of Rock Joints in Theory and Practice. 

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