Traffic Equivalents for Off Highway Recreation Vehicles Using Dirt and Gravel Road

7700 Engineering
April, 2007

Author:
Carolyn Napper, Soil Scientist

Introduction

Significant growth in off-highway motorized recreation has occurred over the past ten years. Many low standard system roads now have as much or more use by "50 in. four-wheel all terrain vehicles (ATVs)" and motorcycles as they receive from ordinary light vehicles. The use and potential impact to forest road maintenance needs is so significant that equivalency factors for this traffic in maintenance sharing calculations is needed.

Currently no factors exist and the ATV and motorcycle traffic is not being included in calculations. Additionally the Forest Service has issued a final rule restricting all motor vehicle use on National Forest Lands to designated roads and trails. It is anticipated that there will be a shift from cross country use to existing low standard system roads. Equivalency factors for ATVs and motorcycles will be used in advising local decisions regarding motorized route designations where there are concerns about appropriated road maintenance funds being insufficient to maintain roads for traffic.

The end product of the T&D work will be equivalency factors for various types of OHV uses and ordinary light highway-legal vehicles uses for making commensurate share calculations. At a minimum, the road types should include earth and gravel.

Existing Direction

The National Forest Roads and Trails Act of 1964 require commercial users of the NFS roads to perform maintenance made necessary by their use. In order to calculate commensurate shares, it is necessary to have equivalency factors between heavy commercial vehicles (i.e. – 18 wheel 80,000 lb GVW log truck) and light vehicles (i.e.- standard pickup truck). Standard equivalency factors have been developed and were latest updated in the early 1990's and are currently included in the model national cooperative road construction and use agreement between the Forest Service and our "cost share" cooperators (FSH 1509.11-95-1 24- Exhibit 01 pg 9 of 17). The same factors are also used for calculating commensurate share for timber sale and land stewardship contact road maintenance packages.

There are numerous manual and handbook references that explain and define the current traffic equivalents determination. The following provides a synthesis of the existing direction.

Direction within the FSM 7700 Transportation System Chapter 7730 Operation and Maintenance defines the following:

7732.12—Investment Protection—Subject to availability of funding, maintain all roads to a condition that protects the Government investment unless an economic analysis determines that deterioration and future reconstruction is more cost-effective.

7732.2—Commensurate Maintenance Responsibility—Assign commensurate maintenance responsibilities to commercial users of National Forest System roads.

7732.21—Maintenance by Forest Service—The Forest Service is financially responsible for road maintenance resulting from traffic associated with:

1. National Forest System land administration.
2. Special-use areas not operated as a commercial enterprise.
3. Incidental non-commercial use related to ownership or occupancy of isolated parcels of private land served by a National Forest System road.
4. Recreation traffic not associated with a commercial activity.
5. Incidental public use.

When a road is being used for commercial purposes, limit Forest Service financial responsibility to a commensurate share of those maintenance activities required for the maintenance level assigned to the road before commercial use.

Use appropriate traffic management controls, including use restrictions, to manage Forest Service commensurate share obligations within available funding.

7732.22—Maintenance by Commercial Users

Require commercial users (FSM 7730.5), except those concessionaires operating Government-owned facilities and those performing public services, to perform maintenance commensurate with their use. Users may deposit sufficient funds with the Forest Service in lieu of performing maintenance. See FSM 1584 for direction on using collection agreements.

Collection Agreement FSM 1584

1. FSH 1509.11-95-1 Grants, Cooperative Agreement and Other Agreement Handbook contain information on Cooperative Road Maintenance Agreement. The Cooperative Road Maintenance Agreement describes our expectations for cooperative road maintenance and reinforces our continued commitment to the principles of the Cost Share Program which was established to ensure access needed to meet management objective for both Forest Service and Cooperators' intermingled lands."
   
   
2. Road maintenance – the performance of work activities needed to preserve or protect a roadway including surface, shoulders, roadside, structures, and such traffic-control devices as are necessary for its safe and efficient use to the standard provided through construction, the most recent reconstruction, or other condition as agreed. (FSH 1509.11 1995)
   
   
3. Traffic generated maintenance – work except repair of major damage made necessary as a direct result of, or to minimize the effect of, use and wear by traffic.
   
   
4. Non-traffic generated maintenance- work made necessary as a direct result of normal weathering processes or uncontrollable influences that cannot be attributed to traffic use.
   
   
5. Equivalent unit (EU)- an average light vehicle having four or more wheels such as a passenger car, pick-up truck, or recreational vehicle moving over a given point in either direction.
   
   
6. Cooperative road maintenance agreement identifies that all traffic generated maintenance shall be shared on the basis of proportionate use, calculated on the basis of the number of EU's assignable to each party determined in accordance with the following procedures.
   
   
7. Native and Gravel surfaced roads:
   
   
1. Computations shall be based on local, or other applicable information, agreed to by both parties, from traffic generated maintenance studies and/or cost data.
   
   
2. For those situations where applicable studies have not been completed but where traffic haul and vehicle classification data are available, computations shall be based on the following formula:
   
   

The Equivalent Unit (EU) Calculation

The Forest Service developed the equivalent unit equation in 1968 in "Allocating Road Construction and Maintenance Costs to the Major Vehicle Classes Using the Forest Service Transportation System." The traffic equivalents were updated in the Cooperative Road Maintenance Agreement in 1995 (FSH 1509.11).

The mathematical calculation to determine traffic equivalents follows:

Number of EUs per MBF (gross measure) =

(Loaded truck weight + empty truck weight + support vehicle weight)/ wt of EU Vehicle/MBF

Use a factor of 5 EUs per MBF. A factor of 4,800 pounds shall be used to convert the gross weight of other commercial haul to EUs.

Loaded logging truck= 80,000 lbs.

Empty logging truck = 32,000 lbs.

Support vehicle=8,000 lbs. (6 pack)

Equivalent vehicle = 4800 lbs.

Therefore, 120,000/4800 = 25 EU's per loaded truck

25 EU's/5 MBF per truck = 5 EU's per MBF.

FSH 7709.58 Transportation System Maintenance Handbook

Incorporates direction formerly in interim directive 7709.58-94-1 on the use of the Cooperative Road Maintenance Agreement (FSH 1509.11, sec 32.1)

Chapter 13—Maintenance Sharing

13.1—Principles—Share financial responsibility for maintaining forest development roads with cooperators, local governments, road users (commercial and noncommercial), or a combination thereof, commensurate with road use. See FSM 7730 for policy.

Allocate financial responsibility for costs associated with management of the forest development road system (road management objectives, condition surveys, maintenance prescriptions, traffic counting, maintenance planning, and other associated activities) to the Forest Service except that the financial responsibility for work made necessary by specific users or groups of users may be allocated to those specific users or groups of users. Other financial responsibilities depend, to some degree, on the type of traffic using the road.

Traffic Types

1. General Use Traffic
2. National Forest Traffic
1. Forest Administrative Traffic: FS
2. Forest Commercial Traffic:Commercial User
3. Forest Visitor Traffic: FS
3. Non-FS Commercial traffic
4. Cooperator (Cost Share)traffic: In accordance FSM 5460

13.2 Sharing Requirements. The legal basis for requiring road users to share in the maintenance of forest development roads is found in 36 CFR 212.7. Implement these sharing requirements through contracts, permits, and agreements as described in:

1. Timber Sale Contracts B (T) 5.4 FS 2400-6 TSC has timber purchasers responsible for road maintenance commensurate with their use.
   
   
2. Cost share Agreements (FSM 7732.24) - Cooperatively maintain the cost share road system to the agreed upon maintenance level in a timely manner to the standard of original construction or reconstruction or as otherwise agreed to in writing. Develop signed maintenance agreements with each cooperator in accordance with the Cooperative Road Maintenance Agreement in FSH 1509.11 section 24.
   
   
3. Road Maintenance Agreements. Implement maintenance agreements with State, county, or local government agencies, and other Federal agencies to document the sharing of road maintenance responsibilities. FSM 1530 has policy for sharing between Federal agencies. FSH 1509.11, Grants and Agreements Handbook, Chapter 30, has direction and sample agreement for cooperative maintenance activities with States, counties or local governments.
   
   
4. Road Use Permits. Include appropriate road maintenance requirements in Road Use Permit, Form FS-7700-41, where authorizing use of existing forest development roads where unrestricted road use has been limited by a road order pursuant to 36 CFR 261.54 and where use is not covered by contact, authorization or agreement.

13.4 Commensurate Sharing—A knowledge or road users, types, classes, use season, and volumes of use for each route is necessary to properly access responsibilities of maintenance.

13.41 Responsibilities and Associated costs.

Item 2 - Roads Assigned to maintenance Levels 3, 4, and 5. Generally a mixture of commercial and noncommercial traffic exists on these roads.

1. Routine maintenance. The financial responsibility for all routine maintenance is shared by the FS and other users. Determine each user's financial responsibility on the basis of traffic units generated by their use. A traffic unit is defines as the average weight of a light, noncommercial vehicle having four or more wheels, passing a given point on a road, moving in either direction. Assign all other vehicles an equivalent traffic unit based on the ratio of the weight of each vehicle to that of the light, noncommercial vehicle (4800 lbs).

ATV and Motorcycle weights

Type of Vehicle	Dry Weight	Overall Width	Towing or Rack Capacity
Honda 2007 TRX 300 EX	372.5 lbs	43.5	N/A
Honda 2007 TRX 450 R	350 lbs	46.3"	N/A
Honda 2007 FourTrax Foreman Rubicon	600 lbs	46.8"	N/A
Honda 2007 FourTrax Rancher 4x4	560 lbs	46.1"	N/A
Kawasaki ZZR600 motorcycle	377 lbs	28.7"	N/A
Kawasaki Brute Force 750 4x4	604 lbs	46.3"	Towing = 1,250 lbs
Kawasaki Bayou 250	404 lbs	40.2"	Towing = 450 lbs
ARCTIC Cat 650 H1 Prowler XT	1160 lbs	61"	Rear rack capacity = 600 lbs. Towing = 1500 lbs
ARCTIC Cat700 EFI 4x4	735 lbs	47"	Towing = 1050 lbs; Front rack = 100 lbs Rear rack = 200 lbs

The increased use of ATV's and motorcycles on Forest Service roads may require cost sharing for the maintenance. Existing direction FSH 7709.58 requires identifying the ratio of the vehicle weight to the equivalent unit (EU) for a noncommercial vehicle (assumed to be 4800 lbs). Using this formula the following EUs could be assigned to the above ATVs and motorcycles.

Light ATV or motorcycle (350 lbs) = 1/13th of EU

ATV 4 X 4 (600 lbs) = 1/8th of EU

ATV (Prowler 1160 lbs) = 1/4th of EU

The values do not reflect the additional weight of a rider. By adding a 150-200 lb rider the values would range from 1/8th to ½ of an EU (assuming two riders on the ARCTIC CAT 650).

Alternate Measures for Traffic Equivalents

There are alternate measures used to estimate the serviceability, or design requirements for roads, flexible pavements, and highways. Basically all measures are developed from models that look at axle configurations which include:

• Wheel base width
• Tire Pressure
• Axle Spacing
• Axle Width
• Dual tire spacing

These parameters affect the loading of a given vehicle making each vehicle slightly different from another. Additionally the response of the road to the load is dependent on road materials, and climatic conditions. Forest roads vary tremendously in terms of the road material. The overall condition of the road depends on its maintenance history, road design, environmental factors (severity of storms), and traffic.

What are Equivalent Single Axle Loads (ESALs) and what do they do?

In 1993 FHWA Pavement Design Guide the use of Equivalent single axle loads was promoted as a common tool to convert wheel loads of various magnitudes and repetitions (“mixed traffic”) to an equivalent number or standard. (Hawaii asphalt/06). This approach is commonly used in the design of paved roads to evaluate the potential deterioration and wear of the road surface. The goal behind ESALs is the same as the Forest Service's Equivalent Unit approach. The standard load for the ESAL calculation is an 18,000 lb equivalent single axle load. From this method, all loads are converted to an equivalent number of 18,000 lb. single axle loads, which is used for road design. The load equivalency factor for a car is .0007, a 2,000 lb. single axle is .0003 and a delivery truck is .10. When spreading a load out over two closely spaced axles reduces the number of ESALs, therefore for an ATV the ESAL would be much lower.

What is Axle Load Spectra?

A newer model called the Axle Load Spectra was included in the 1-37A guide (FHWA Pavement Design Guide). The Axle Load Spectra model characterizes loads directly using truck speed, gear/axle configuration, axle/tire spacing, tire pressure, traffic wander, and monthly/daily distribution factors. The load spectra analysis does not use equivalent values, instead traffic load applications for each truck class and axle type are directly characterized by the number of axles within each axle load range. Load spectra is more complicated yet potentially more accurate in its load characterization than ESALs. Axle load data is collected at Weight in motion (WIM) stations to encompass the principal truck axles on the roadway network. A WIM station is a permanent device that continually collects and stores axle weight data. The device also collects the total number of vehicles, axle spacing, length, speed, and vehicle type data.

Both ESALs and Load Spectra require traffic characterizations for each vehicle. Estimation of the amount and type of traffic that the roadway will be expected to carry over the design of the structure and its predicted pavement performance. Traffic analysis is an essential part of analysis and sizing of facilities. (Skok 2003) Therefore the percentage of each class of vehicle or ESAL within the overall traffic must be determined.

Conclusion

The Forest Service could use any of the above methods to assign traffic equivalents for all vehicles using system roads. Since all methods use a type of traffic equivalent or load basis, the value assigned to an ATV or motorcycle will be a small percentage of a noncommercial recreational vehicle. However, what may not be captured by such a figure is how the road is used, and the amount of traffic a given area may receive over a relatively short period of time. For example, motorcycle use on system roads may include riding on cutslopes. The effect of this on sediment production and ditch maintenance may require more frequent maintenance. ATV use on roads can result in berm development from banking along curves.

Season of use may result in the most severe impacts to both on-site and off-site resources. ATV use during the opening weekend of deer hunting season on a forest may have high traffic use over a relatively small area. The use may occur at a time where forest roads are wet and subsequent impacts may be great to not only the road (degradation of design drainage) but to off-site resources including water quality. The EU assigned to a motorcycle or ATV may not reflect that impact, however the same is true of wet-season haul by logging trucks.

The most difficult aspect of the commensurate share for ATVs and motorcycles will be in obtaining accurate traffic counts of activities, rather than the actual EU. Accurate traffic counts will require counters to be set up on each road within the ATV riding area. The seasonality of the traffic will also have to be captured. Finally, given the growth of ATV riders, the change in riding use on system roads over time will have to be tracked.

Motorcycle impacts to cutslope on the San Bernardino NF R-5.


This road surface and drainage design has been modified from driving ATVs on the cutslope.