CHAPTER 10

Autonomous Mowers

10.1 Description of Autonomous Mowers

An important aspect of roadside maintenance, vegetation control, helps clear sight lines along a roadway so that drivers are able to see traffic control devices, pedestrians and bicyclists, and wildlife or livestock. It also provides adequate sight distance for intersection approaches and throughout curves (Eck and McGee 2008). In areas that are prone to fires, such as California, proper vegetation control can reduce fire risk. Additionally, appropriate vegetation control can reduce the risk of tort liability lawsuits (Eck and McGee 2008).

Traditional roadside vegetation control strategies generally include a mixture of methods such as mechanical control (e.g., mowing and trimming), chemical control (i.e., herbicides), permanent control (e.g., paving, laying gravel), cultural control (e.g., mulching, burning, flooding), and biological control (e.g., grazing). While chemical control has been used frequently, many agencies are reducing their use of herbicides due to environmental and health concerns. This has increased the reliance on mechanical control, which is labor-intensive and expensive.

One of the major drawbacks to mowing operations is the need for equipment operators to operate on the roadside, which introduces safety risks for both operators and the traveling public. One way to mitigate some of this risk is to use teleoperated (particularly when line of sight is not required) or automated mowers that remove human operators from the machines.

Two types of automation have been used for mowers. First, fully autonomous mowers are available for residential use and typically utilize boundary wires or geofences to establish boundaries (Hicks and Hall 2000, Husqvarna 2022). Second, some agencies have used remote-controlled mowers. These are still controlled by a human operator, but operations can take place at a distance.

10.2 Examples of Applications for Autonomous Mowers

Information about how agencies have utilized autonomous mowers was gathered from a review of the literature and a survey of agencies.

10.2.1 Responses from the Survey

A survey was conducted to gather information about the automated processes that IOOs have implemented or are planning to implement, as described in Chapter 3. Agencies were asked about the automated processes that they have used or piloted and the processes that they thought

could benefit from automation. Eighteen percent (n = 6) reported using autonomous or remote mowers, and 35% (n = 12) indicated that they planned to use or pilot the use of these technologies in the next 3 to 5 years. Agencies were also asked about the processes that they thought would benefit the most from automation. Two-thirds (66%, n = 19) felt that mowing and vegetation control processes would benefit from automation.

10.2.2 California

Autonomous Mower Applications

Caltrans evaluated the use of commercial autonomous mowers through a series of projects completed by the University of California, Davis. The initial research developed a test bed roadside mower and a set of initial specifications to meet Caltrans’ needs (Arsenault et al. 2008, 2010, 2011). The mower specifications included the following:

• Never leave the set boundary (i.e., do not encroach into the roadway),
• Be cost-effective,
• Be easy to set up and operate,
• Have a rate and quality of mowing comparable to that of human-powered mowing,
• Be relatively small,
• Be reliable and easy to repair, and
• Be able to avoid obstacles.

The initial test mower was 231 lb, 40 in. long, and 38.3 in. wide and had a 34.5 in. track. The operating speeds were up to a maximum speed of 3 mph. The mower had a low-cost sensor array and GPS to maintain positioning. A field trial indicated that the mower could attain a mowing consistency of 4 in. (Arsenault et al. 2008, 2010). Based on the test mowing rates, a cost savings of around $230,000 per year for District 3 was estimated assuming the use of autonomous mowing for 25% of the total acreage. The test mower had problems with mowing consistency and was not able to avoid obstacles.

Creed et al. (2011, 2012) evaluated a modified version of the test mower from the original Caltrans evaluation. A laser range finder and tracking controller were added to detect-and-avoid obstacles. Tests showed that the modified test mower was able to successfully detect and navigate around the obstacles while in autonomous mode, but the researchers felt that the system was not yet ready to deploy on roadsides until additional safety functions were added.

Hasegawa and Wians (2018) converted a commercial self-driving mower to use lidar technology. The lidar scanner was able to detect fences, tall grass, and ground levels.

Remote Mower Applications

A recent study for Caltrans evaluated the use of commercially available remote-controlled mowers (White and Lasky 2019). The Alamo Traxx and Green Climber remote-controlled mowers, which are capable of mowing otherwise non-traversable slopes, were tested to determine mowing rates as well as the maximum slopes they were able to traverse. Operations costs were also assessed, and the authors report that the use of automated mowers was more expensive (275%) than the use of traditional mowers, but less expensive (75%) than the use of a string trimmer. The main drawbacks were significant downtime for repair and difficulty obtaining parts.

An especially wet winter in California in 2022–2023 led to an abundance of grass on the roadside, which frequently can lead to grass fires. Caltrans is using remote-controlled bush cutters in Caltrans District 6 to reduce the hazards of grass fires. The technology is used in areas that cannot be accessed by heavy vehicles. Using these remotely controlled vehicles also increases the safety of maintenance crews and reduces trip and fall hazards (Nemarich 2023).

10.2.3 Florida

Remote Mower Applications

The Lehigh Acres Municipal Improvement District in Florida is using a remote mower along 300 mi of canals to control erosion, maintain drainage systems, and ensure safe passageways. The district noted that manual mowing along canals had only been done 3 times a year and was labor-intensive. Additionally, workers were exposed to injuries from falls as well as snake bites. The remote-controlled mowers have improved work safety and increased efficiency.

10.2.4 Georgia

Remote Mower Applications

The city of Atlanta, Georgia, is piloting the use of remote-controlled mowers. The city tested RC Mower units at a water reservoir and is considering using the mowers as a means to reduce costs and increase productivity. The city is considering purchasing three additional remote-controlled mowers (Nobles 2022).

10.2.5 Indiana

Autonomous Mower Applications

The Indiana DOT recently (August 2022) started a pilot project to evaluate the use of autonomous mowers. The project will collect machine data from current equipment driven by human operators, develop autonomous algorithms and virtual simulations, and perform physical algorithm validation on an autonomous trimming platform (JTRP n.d.).

10.2.6 North and South Carolina

Remote Mower Applications

Contractors use robotic mowers to maintain the Interstate areas of North Carolina and South Carolina. The mowers used in these activities can be remotely controlled from 1,000 ft away. A contractor stated that the use of robotic mowers has increased the efficiency of operations by 25% and cut labor costs in half. These mowers have proven to be especially effective on steep terrain because the workers do not need to climb to reach difficult areas (Wilson 2015).

10.2.7 Ohio

Remote Mower Applications

The Ohio DOT uses remote-controlled mowers named Flailbots in District 4. These mowers can safely cut grass in steep areas that are dangerous for tractors due to the risk of rollover. These mowers also increase the safety of workers by avoiding the need for human operators along highways and ramps (Marotta 2021).

10.2.8 Texas

Remote Mower Applications

Several Texas agencies are currently using remotely controlled mowers in limited settings. The Transportation Division of Midland, Texas, is using remote mowers for steep inclines and other hard to reach areas. Portland, Texas, is using remote-controlled mowers to maintain city parks and drainage ditches (RC Mower 2022).

10.2.9 Other Autonomous Mower Technologies and Applications

Agencies have evaluated the use of other types of automation technologies and applications for mower operations.

In a pilot project, Minnesota acquired 30 traditional mowers in 2016 that it then equipped with AVL to track where vegetation control has taken place and optimize mowing routes. The pilot project leveraged previous work with AVL in mowers to develop a software interface, create a data exchange, purchase and install the AVL systems, develop a reporting system for the collected data, and provide training in the use of the technology (Potter and Bayer 2017). The pilot project found that the use of AVL-equipped mowers has the potential to save MnDOT on operational costs, optimize mower routes, reduce paperwork, and reduce the spread and exposure risk of noxious weeds.

Researchers at Purdue University explored the impact of automated mowers on airport sustainability. Airports consist of hundreds to thousands of acres of land, and the air traffic system can be greatly hampered by birds or other wildlife in these areas. It was found that automated mowers are an efficient means to improve airport sustainability by reducing the number of workers, using solar power, and cutting gasoline use (Hubbard et al. 2021).

Irizarry et al. (2022) evaluated the use of UAVs to automate the planning, monitoring, and performance certification of highway maintenance tasks for GDOT. Currently, GDOT manually verifies the performance of mowing contractors, which is labor-intensive. The researchers developed a machine learning-based grass mowing assessment framework that utilized UAV image data. The developed system successfully identified mowed versus unmowed grass areas. Additional research is needed before the system is operational.

10.3 Summary of Applications for Autonomous Mowers

Two states have piloted autonomous mowers, while six are testing or using remote mowers.

10.3.1 Advantages

The main advantages of the use of autonomous or remote mowers include the following:

• Reduced spread of noxious weeds along roadways,
• Reduced herbicide/pesticide use,
• Lower operational costs,
• Reduced safety risk for field crews,
• Ability to optimize mowing routes, and
• Increased efficiency.

One of the most beneficial advantages of autonomous or remote mowers is their ability to operate on steep or other challenging terrain, where their use can prevent falling injuries for workers.

10.3.2 Disadvantages

The main drawback to the use of automated mowing technology is that no autonomous mower currently has the capability of independent function along an active roadway. Other drawbacks include concerns that autonomous mowers may encroach on the shoulder or roadway, or create a distraction for drivers.

10.3.3 Costs

Most of the sources consulted did not provide specific estimates of cost.

10.3.4 Status for Autonomous Mowers

No DOT is currently utilizing autonomous mowers for roadside vegetation control. However, the technology is available for residential use, and several agencies have tested the technology. As a result, while the technology exists, policy or technological barriers hamper adoption.

Several agencies are using remote-controlled mowers and are using UAVs to verify their performance.