TECHNICAL INFORMATION, RESEARCH, AND EDUCATION (TIRE) COMMITTEE ADVISORY FOR ZIP LINE BRAKE SYSTEMS
Author: ACCT Technical Information, Research, and Education (TIRE) Committee
Disclaimer: The Association for Challenge Course Technology (ACCT) seeks to advance the industry and enable members’ ongoing success. This document is a work product of the Technical Information, Research, and Education (TIRE) committee and represents a consensus of members of the TIRE committee. The content of this document is not a new ANSI/ACCT standard, nor does it alter or interpret an existing ANSI/ACCT standard, and may not represent the official position of the ACCT.
Zip lines have a long history in the challenge course field and are often part of a larger participant experience. Since the early 2000s zip lines became increasingly popular with types ranging from low-speed aerial adventure park and guided canopy tour zip lines to high-speed, long-distance rides. Matching the zip line brake system with the arrival speed of the rider (participant or staff) is critical to minimizing risk of injury (Speelman, Wagstaff, Jordan & Haras, 2021). Serious injuries to riders and zip line brake system attendants (staff) have occurred in human-operated (participant and guide active) zip line brake systems, increasing in severity with higher arrival speeds. Smaller and shorter zip lines with arrival speeds of 6 mph (10 km/h) or less do not require brake systems (as gravity takes care of the problem) and the risk of injury is much lower. This discussion excludes this category of zip lines.
The Speed-Risk Relationship
Speed is a key hazard factor in automobile injuries influencing both the risk of injury and its severity. This relationship is particularly significant for pedestrians and cyclists who have no crash protection – a situation similar to zip line riders. For example, the likelihood of pedestrian survival is 90% when struck by a vehicle travelling at 18.5 mph (30 km/h) but drops below 50% at 28 mph (45 km/h; Peden et al.,2004). This relationship explains why most jurisdictions limit the speed in parking lots and on paths shared by non-motorized users to 15 mph (24 km/h; see for example, Adkins, 2014; Department of Conservation and Recreation, 2017). The Technical Safety and Standards Authority (TSSA) in Ontario Canada uses 15 mph (24 km/h) as the upper speed limit for active zip line brake systems (Amusement Devices CAD - 535-18).
Factors to Consider in Brake Systems
Zip line brake systems vary in design and the use of human-operated brake systems is not new (cf. Rohnke, 1977). The following are some of the variables that deserve careful consideration when evaluating whether individuals (both riders and landing platform brake attendants) have been provided with a brake system that limits the potential for serious injury.
Reaction Time and Distance
The distance a zip line rider travels between the point at which the brake attendant recognizes the need to brake and the point at which they begin applying brake force is called the reaction distance. It is directly proportional to speed, meaning that a rider traveling twice as fast is going to have a reaction distance that is twice as long. Reaction time ranges from half (0.5) a second to two (2) seconds and is influenced by operator experience and anticipation (Ottawa Safety Council, 2021). Therefore, experienced zip line guides are better at operating guide-active brake systems than novice guides. The role of experience also explains why participants have proven to be unreliable at primary braking.
The distance travelled by the zip line rider after the brake system has been activated until they come to a complete stop is called braking distance. Unlike the one-to-one relationship we see between speed and reaction distance, when the speed of a rider doubles the braking distance quadruples (with the braking force constant) due to the relationship of speed and related energy that must be absorbed by the brake. This non-linear relationship between rider speed and braking distance becomes particularly problematic if brake attendants misjudge the speed and distance of riders at high speed – the braking distance required to stop a rider at an acceptable rate quickly exceeds the amount of space that is available.
When more braking force is applied to reduce the braking distance, a rider suspended by a pivoting lanyard will experience a pendulum swing due to the momentum created by the zip trolley stopping but the rider continuing to be in motion. On primary brakes, riders should come to a stop somewhat gradually and comfortably with a pendulum swing that will not allow them to contact the zip cable or any brake components. Some contact with objects may be acceptable in emergency braking.
The following is suggested when designing or using human-operated zip line brake systems:
- Due to the inability of staff to physically intervene with participant actions when a participant is riding a zip line, combined with the risk of injury if the participant fails to properly apply the brake, it is prudent to design zip line brake systems that do not depend on participant actions to provide the primary brake. Participant interaction must be backed up on lines with arrival speeds above 6 mph (10 km/h) – a situation directly comparable to what occurs when participants are belaying, rappelling, or engaging in other safety critical tasks.
- Evaluate the environmental conditions in which the zip line operates, with special consideration given to wide variations in conditions that may affect landing speed on a given zip line. These include wind speed and direction, temperature, humidity, and precipitation. Consider whether an adequate margin for error (buffer) exists to account for these variables during operation over the course of a whole season that were not present at the time of commissioning (aka operational testing). In short, avoid designing zip line brakes that must function at their operational limit with no margin for error.
- Consider implementing a systematic gathering of data from pre-use checks and periodic monitoring to determine if the predicted rider speed continues to be accurate or is increasing over time, and therefore whether each zip line brake system continues to properly function. Plan to review this documentation on a regular basis.
- Ascertain whether brake attendant strength and reaction time are appropriate for the mass and speed of riders on each zip line at the facility. These staff members should not need to possess extraordinary strength to provide an effective brake. The ability of the brake attendant to maintain control throughout the braking process should be considered as it relates to closed loop rope brake systems.
We strongly urge all those with any connection to human-operated zip line brake systems to evaluate rider speed on entering the brake zone, with a specific focus on reaction distance, experience profile of the operator (brake attendant), and total braking distance. Higher speed zip lines require different brake systems than lower speed zip lines and the highest speed zip lines require automatic (passive) primary brakes.
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Adkins, C. (2013). Five Rules for Parking Lot Etiquette. National Motorists Association Blog (13 November). https://ww2.motorists.org/blog/rules-for-parking-lot-etiquette/
Amusement Devices Code Adoption Document – Amendment 535-18. Toronto, ON: Technical Standards and Safety Authority. https://www.tssa.org/en/amusement-devices/resources/Documents/AD- 535-18-CAD-Amendment.pdf
Department of Conservation and Recreation. (2017). In MassDOT Design Guide. Shared Use Paths and Greenways (chapter 11). Commonwealth of Massachusetts. https://www.mass.gov/doc/massdot-design-guide-chapter-11-shared-use-paths-and-greenways
Ottawa Safety Council. (18 March 2021). Stopping distance + braking safely. Ottawa, ON: Author. https://www.ottawasafetycouncil.ca/stopping-distances-and-distracted-driving/
Peden, M., Scurfield, R., Sleet, D., Mohan, D., Hyder, A.A., Jarawan, E. & Mathers, C. (Eds.). (2004). World report on road traffic injury prevention. Geneva: World Health Organization. https://www.who.int/publications/i/item/world-report-on-road-traffic-injury-prevention
Rohnke, K. (1977). Cowstails and Cobras: A guide to ropes courses, initiative games, and other adventure activities. Hamilton, MA: Project Adventure.
Speelman, E.A., Wagstaff, M., Jordan, S.H., & Haras, K. (2021). Aerial adventure environments: The theory and practice of the challenge course, zip line, and canopy tour industry. Champaign, IL: Human Kinetics.