Agricultural Equipment on Public Roads

  • Committee on Agricultural Safety and Health Research and Ext,


Preface, Acknowledgments, Executive Summary
1.0 Introduction
2.0 The Rural/Urban Traffic Interface
3.0 Federal and State Regulations
4.0 Higher Speed Tractors
5.0 Transportation of Workers on Public Roadways with Farm Equipment
6.0 Suggestions for the Future
7.0 References
8.0 List of Committee on Agricultural Safety and Health Research and Extension Members



  • In the United States, farm tractors, self-propelled and towed equipment often fall outside road vehicle legislation requirements.
  • Tractor speeds have increased in recent years. In order to protect other road users, tractors and towed equipment must be engineered to allow the driver to retain control of both the tractor and towed equipment under all conditions.
  • Key safety-related systems that may be an issue at higher speeds include steering, brakes, suspension, tires, alignment, hitching components, tractor rollover protective structures (ROPS), SMV emblem, and the speed indicator symbol (SIS).



Historically, the majority of tractors in the United States were designed to travel at a top speed of approximately 20 mph (30 km/h). These vehicles normally featured rigid rear axles and trunnion mounted front axles with full engineering standards available for design and manufacturing processes. In the 1980s European tractors, particularly those with 100 hp (75 kW), began to be designed with a top ground speed exceeding 25 mph (40 km/h). Physically, these tractors are similar to 20 mph (30 km/h) machines except in gearing and brakes. Tractor standards acknowledged their presence and were modified to incorporate appropriate braking standards. Tractors incorporating higher ratio gearing and suspension of their front axles were introduced in 1994; these tractors were able to travel 32 mph (50 km/h). In 2005 ASABE Standard S390, “Definitions and Classifications of Agricultural Field Equipment”, was revised to include categories of ground speed. The standard, also approved by the American National Standards Institute (ANSI), divided agricultural field equipment into 5 ground speed classifications (Table 3) based on their nominal maximum ground speed in an original equipment configuration as designed and specified by the manufacturer. While there is no specific definition of “higher speed” tractor in this standard, for our purposes, when a tractor’s highest speed is rated equal to or more than 25 mph (40 km/h), the tractor is considered as a higher speed tractor.

Table 3. Agricultural equipment ground speed classes

Ground Speed (km/h)
Agricultural Field Equipment Group

Agricultural tractor






ATR65 Plus

Towed implement






ATI65 Plus

Rear mounted implement


Not Applicable

Rear mounted implement


Not Applicable

Rear semi-mounted implement






SMR65 Plus

Front semi-mounted implement






SMF65 Plus

Self-propelled machine






SPM65 Plus

Bulk carrier/agricultural trailer






ABC65 Plus

Source: ASAE Standard 390.4 (2005)

A review of the Nebraska Tractor Test Summary reports shows that of over 500 tractors tested (Grisso, 2007), road gear speed of the tractors tested has increased in the last five years (Figure 1). Currently between 40-45% of the tractors tested are equal to or exceed 25 mph (40 km/h). When compared to the tractors tested over the last 20 years to the last five years (Figure 2), there is an increase in tractors tested at speeds equal to or greater than 25 mph (40 km/h). The results indicate that tractors are available that can exceed 25 mph (40 km/h).

 Figure 1. The frequency of the tractors tested by year that the high-gear allowed for travel speeds greater than or equal to 25 mph.

 Figure 2. The percentage of tractor high gear speed for road transport tested over last 20 years and tested the last five years.


Historically, tractors have incorporated: a) pure mechanical steering; b) hydraulically assisted mechanical steering; and c) full hydrostatic steering systems. While many older tractors still in use have mechanical steering, most current tractors use hydrostatic steering. The characteristics of hydrostatic steering are:

  • Low steer effort
  • High steer torque
  • Limited or no feedback from the road wheels to the steering wheel
  • Limited or no self aligning ability, and
  • Limited steering in the event of an engine or hydraulic failure.

Loss of steering during an engine failure has been a concern but tests show that during these situations, total malfunction of the steering system does not occur (Grisso, 2007). The driver is able to steer the tractor within a determinate radius and has time appropriate for stopping the tractor. Experience shows that hydraulic steering systems do not fail abruptly. In addition, some systems are self-aligning and are designed with sufficient hydraulic reserve for the driver to respond appropriately and maintain control of the tractor.

The response of the vehicle to input from the steering wheel is critical to vehicle feel and behavior. If the time is too short, the tractor will be sensitive to operate and require continuous correction to maintain it in a straight line. Conversely, if the time is too long, the tractor will be sluggish to respond and may create steering problems for the driver. In the extreme case, if the driver first steers right and then rapidly left (as they would while driving a car), the driver may be turning the wheel left while, or even before, the vehicle has started to move right. Or the driver may continue turning right, resulting in turning too far right. In either case the steering wheel becomes out of phase with the motion of the road wheels and in attempting to correct this, the steering column can appear to have elastic properties. In practice, the target response time to develop maximum cornering force is between 0.6 and 0.8 seconds.


Fundamentally, brakes serve the function of reducing vehicle kinetic energy by conversion into heat energy. As a function of the square of vehicle speed, kinetic energy increases rapidly. For example, a tractor traveling at 50 mph (80 kph) dissipates approximately seven times the energy for braking than a tractor traveling at 20 mph (30 kph). This situation is exacerbated by the legal requirement for faster moving vehicles to decelerate at higher rates. For example, 20 mph (30 kph) tractors have historically been required to have braking systems capable of deceleration at 9.3 ft/s2 (2.8 m/s2). When tractors reach a speed of 30 mph (50 kph), they are required to decelerate at a rate of 16.4 ft/s2 (5.0 m/s2), which is the same as the trucking industry.

With the combination of higher energy level and more rapid deceleration, brake systems with excellent heat dissipation characteristics are required. Conventional tractors have normally relied on either dry or oil immersed disc brakes incorporated within the tractor rear axle. The oil used is common with that used for axle lubrication, gearbox lubrication and as an external hydraulic oil supply to implements. Contamination of this oil with brake lining debris can lead to serious functional problems within the tractor hydraulic or transmission systems. Breakdown of oil lubrication properties can also occur if the oil is subjected to high temperatures leading to impaired durability of components.

The weight distribution and large rear tires of conventional tractors have enabled tractors to generate sufficient braking effort from their rear wheels alone; typically such tractors have no front brakes fitted. The move to 25 mph (40 kph) tractors in Europe has coincided with the almost universal acceptance of front wheel assist driven axles. This has given manufacturers the opportunity to engage the front axle drive while braking. This technology has also been carried into the 32 mph (50 kph) tractor models, with the addition of incorporating some form of disc brakes onto the front drive system to assist the braking effort.

According to the ANSI/ASAE Standard, S365.8, “Braking System Test Procedures and Braking Performance Criteria for Agricultural Field Equipment,” the braking system requirements for agricultural trailers and towed agricultural machines are broken into two areas: one concerning towed equipment without brakes and the second with brakes:

  1. For towed equipment WITHOUT brakes, the following information shall be provided: Do not tow equipment that does not have brakes:
    • at speeds over 20 mph (32 kph); or
    • at speeds above that recommended by the manufacturer; or
    • that, when fully loaded, has a mass (weight) over 3300 lb (1.5 t) and more than 1.5 times the mass (weight) of the towing unit.
  1. For towed equipment WITH brakes, the following information shall be provided: Do not tow equipment that has brakes:
    • at speeds over 32 mph (50 kph); or
    • at speeds above that recommended by the manufacturer; or
    • that, when fully loaded, has a mass (weight) more than 4.5 times the mass (weight) of the towing unit.
    • at speeds over 25 mph (40 kph), when fully loaded has a mass (weight) more than 3.0 times the mass (weight) of the towing unit.


United States tractors are not traditionally manufactured with suspension systems. However, a fully suspended chassis, i.e., a suspension system for both front and rear axles, may improve handling at all speeds. On a conventional tractor without suspension, the weight can come off the wheels when going over a bump, giving minimal traction when brakes are applied. The weight is also transferred forward onto the front axle, but most of the braking power is in the rear axle. These factors combine to limit the braking ability of the conventional tractor. With a full suspension, as the wheels go over bumps in the road the wheel and axle are able to move up out of the way of the rough terrain while the weight distribution remains similar. With a full suspension the wheels are more apt to stay in contact with the ground which will maximize the traction coefficient of the wheels during braking and under traction. For example, a full chassis construction allows the mass of the machine to ‘float over’ the full suspension while the axles follow the contours of the ground.

In general, the benefits of a full suspension system can be summarized as follows:

  • Greater ride comfort and isolation from whole body vibration, both in the field and on the road.
  • Better control of the vehicle by the driver through minimized ground force variations of the wheels.
  • Better handling characteristics of the vehicle for safer use on the road, particularly at higher speeds
  • Increased traction through constant ground force at the wheels.
  • Potential for greater travel speeds made possible by minimized body accelerations.

The requirements for an optimal full suspension system on a tractor, whether higher speed or conventional, are:

  • Tires kept such that the force that they exert on the ground surface remains nearly constant.
  • Tractors able to experience large variation in loading either within the wheelbase (as in a loaded truck) or cantilevered at the rear or front of the vehicle when carrying mounted implements.
  • During high power and high draft operations, power is transmitted through the drive wheels using low speed and high torque. This torque has to be reacted through the axle location mechanism with no vertical component reaction.
  • Significant axle travel to avoid generating high ground forces when addressing bumps
  • Predictable and controllable cornering characteristics are most easily achieved with equal tire sizes on both front and rear axles.


Road transport is one of the extreme uses for an agricultural tire because a tire's worst enemy besides the hard pavement is heat. The recommended pressure not only depends on tire load (carried by the axle) but will depend on maximum speed. Different load/inflation tables are developed for the maximum speed of the machine. Tire data books list weight capacities and recommended air pressures along with maximum travel speeds.

According to the ANSI/ASAE Standard, S430.1, “Agricultural Equipment Tire Loading and Inflation Pressures,” agricultural type tires are not designed for highway vehicle use or to operate at speeds in excess of 25 mph (40 km/h) except for the F1 tires designated as highway use. For agricultural tractor tires, according to SAE J709, similar designations are warranted for higher speed travel.


The rollover protective structure (ROPS), as described in the Society of Automotive Engineers (SAE) Standard J2194 “Roll-Over Protective Structures (ROPS) for Wheeled Agricultural Tractors”, is a protective structure designed to minimize the frequency and severity of operator injury resulting from accidental tractor overturn. ROPS are designed to absorb energy resulting from the impact of the tractor with the ground surface during a tractor overturn. The intent of the standard and the testing procedures, according to SAE J67 “Overhead Protection for Agricultural Tractors—Test Procedures and Performance Requirements” is to protect the operator during field operations and not for vehicle crashes. The current ROPS test standard limits tractor test speeds to 3-5 mph (5-8 km/h) for rear rollover, and a minimum velocity of 10 mph (16 km/h) for side rollover.

Liu and Ayers (2007) reported on the following concerns for ROPS on a higher speed tractor: 1) how much more energy should a ROPS for a higher speed tractor absorb; 2) how different are the impacting forces that the higher speed tractor will generate if it overturns; 3) how the forward speed influences the energy absorbed by the ROPS in the longitudinal and vertical directions; and 4) if the current criteria for the ROPS test is compatible or strong enough for the ROPS of the higher speed tractor. They did not address safety trade-offs that a stronger ROPS may introduce, such as lowered operator visibility (especially when entering roadways), the potential of decreased stability from a higher center of mass, or the increased risk to other road users from higher mass tractors.


The North American drawbar hitch is a uniquely designed hitch and may not be adequate for higher speed tractors. The drawbar and hitch pin configuration may give too much flexibility for stable control at higher speed. A ball hitch (80 mm being considered as a standard) would be an effective solution but the location of the ball relative to the tractor rear axle is critical. The farther forward the hitch is connected, the more stable the towed equipment will be during road operation. Unfortunately, moving the hitching location forward decreases the turning radius, which limits operations during fieldwork.

This document is from the
North Central Education/Extension Research Activity Committee 197 Cooperative State Research, Education, and Extension Service United States Department of Agriculture

Recommended citation: Committee on Agricultural Safety and Health Research and Extension. 2009. Agricultural Equipment on Public Roads. USDA-CSREES, Washington, DC.

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