Did you know the City of Ottawa is currently in the midst of its fourth electric scooter, or ‘e-scooter,’ pilot program? E-scooters have become increasingly popular as a fun and convenient means of transportation, and they are now available in the Nation’s Capital, with new restrictions on riders as well as changes to the scooters themselves[1].

Whether you’re a cyclist, pedestrian, or e-scooter rider, you are a ‘vulnerable road user’ when compared to motor vehicles and are exposed to the risk of sustaining injuries if involved in a collision. So, what should you know about riding an e-scooter?

Figure 1: E-scooters are rolling out in Ottawa in Phase 3 of its pilot program[1].

Know the Risks of Riding

Research shows that the most common cause of injuries to e-scooter riders is not from contact with vehicles, but from falls!

A study of 105 adults injured while riding e-scooters in Washington, DC, found that approximately 67% of incidents were the result of falls due to adverse surface features or infrastructure (bad roads or rough travelling surfaces). Only 10% of incidents were due to collisions with motorized vehicles[2]. Similar findings were observed in Copenhagen, Denmark, where falls accounted for almost 90% of e-scooter rider injuries[3].

Head/facial injuries were the most common ones sustained by e-scooter riders who suffered a fall in these studies, with less than 4% of riders wearing helmets at the time of their fall. As with cyclists and pedestrians, head–ground impacts are an important determinant of severe head injuries with e-scooter riders.

In Ottawa, e-scooters are allowed on roads with a posted speed limit of 50 km/h or less. The shared e-scooters use geofence technology that enforces ‘no ride,’ ‘no park,’ and ‘slow zones,’ which include transit stations, NCC pathways, the ByWard Market, and City of Ottawa parking garages. The operating speed of the shared e-scooters is capped at 20 km/h, or 12 km/h in the ‘slow zones.’ However, helmets are not required for riders aged 18 or older!

Fall Kinematics

A recent study by Posirisuk et al.[4] completed a computational prediction of head–ground impact kinematics in e-scooter falls. To simulate falls, the authors incorporated potholes of varying widths (20 cm, 40 cm, 60 cm, and 80 cm) and depths (3 cm, 6 cm, and 9 cm) to explore the effects on the head–ground impact force and velocity for a wide range of rider anthropometry (5th percentile female to a 95th percentile male) at various e-scooter riding speeds (10 km/h, 15 km/h, 20 km/h, 25 km/h, and 30 km/h).

The computer-based predictions were validated by comparison to a human volunteer test carried out by the Swifty scooter company[5]. That test involved a volunteer riding an e-scooter at approximately 10 km/h and falling to the ground after entering a pothole 80 cm wide and 7.2 cm deep (Figure 2).

Figure 2: Comparison between the pothole test by Swifty (Allerton, 2020) and computer-based predictions (reproduced from Posirisuk 2022 study).

The scooter fall could be divided into two phases: (1) the contact phase, in which the e-scooter encounters the pothole’s second edge; and (2) the discharge phase, in which the rider falls from the e-scooter and becomes airborne before impacting the ground. The authors observed three different types of fall kinematics, which depended on the pothole depth, width, and e-scooter speed, captured in the following figures:

Type 1: 66% of falls resulted in the rider being discharged forward from the e-scooter, with a body part contacting the ground before head–ground contact.

Type 2: 28% of falls resulted in the rider being discharged from the e-scooter, with the head contacting the ground first.

Type 3: 6% of falls results in the rider being discharged from the e-scooter, with their body rotating to the side before impacting the ground on the side, followed by head–ground impact.

The authors noted that Type 3 falls increased along with pothole depth. All falls were Type 2 when the e-scooter speed was 30 km/h. The authors also noted that none of the riders fell when the depth of the pothole was 3 cm (12% of the e-scooter wheel’s diameter), and that potholes deeper than 6 cm significantly increased the risk of falls.

The City of Ottawa’s recommended maintenance quality standard[6], which applies to the maintenance operations on bridges, paved and treated road surfaces, including cycling lanes, paved and gravel shoulders, defines the treatment for surface distortions that could pose a risk to cyclists and motorists. This standard allows surface distortions, like potholes, to be up to 25 cm in width and 5 cm in depth on roadways and up to 40 cm in width and 8 cm in depth on paved or non-paved shoulders. This means e-scooter riders may encounter surface discontinuities on roadways that may cause a fall to an inattentive, unprepared, and/or inexperienced rider.

Biomechanical Injury Assessment – Head Injuries

The above computer model detailed an increase in head impact speed as e-scooter speed increased. At 10 km/h, the head impacted the ground at an average speed of 4.8 m/s, while at 20 km/h, the head impacted the ground at an average speed of 6.9 m/s.

To assess the risk of injury associated with these impacts, head accelerations can be compared to the Injury Assessment Reference Value (IARV) and probability curves published by Mertz et al.[7] The Head Injury Criterion (HIC) , a measurement that quantifies the severity of a head impact by incorporating its magnitude and duration, can also be calculated and compared to risk curves for severe brain injury (AIS ≥ 4) for the adult population; where AIS ≥ 4 (severe) and above include penetrating skull injuries leading to brain injury, large contusions and hematomas (e.g., subdural, subarachnoid, and intracerebral).

Research by Cripton et al.[8], showed that head impact speeds of 5.4 m/s and 6.3 m/s (i.e., within the range obtained in the above scooter study) resulted in peak head accelerations of 601 to 824 g when not wearing a helmet (Figure 4). Mertz et al.[9] estimated a 50% risk of fracture for an adult skull at a peak head acceleration of approximately 260 g, with head accelerations greater than 500 g resulting in over 99% risk of fracture for an adult skull. Based on the HIC results, it is also noteworthy that the probability of severe (AIS ≥ 4) brain injury was determined to be 99.9% for an unhelmeted 5.4 m/s impact speed (Figure 3)[10].

Figure 3: Calculated probability of sustaining a severe brain injury (AIS ≥ 4) based on the HIC values for unhelmeted and helmeted drop tests (reproduced from Cripton et al.).

Figure 4: Peak head accelerations for both helmeted and unhelmeted drop tests from Cripton et al. An unbraced fall from 1.5 meters to 2 m results in peak resultant head accelerations of 601 to 824 g (outlined in red).

Mechanical Thresholds Associated with Traumatic Brain Injury

The head acceleration levels discussed above can also be compared to the acceleration levels associated with concussion in the peer-reviewed literature. For example, Brennan et al.[11] completed a systemic review of studies that published objectively measured biomechanical forces associated with a concussive event. This included 13 studies which documented American football athletes, at either a high school or collegiate level, and unhelmeted amateur rugby athletes. The average linear acceleration associated with concussion reported by Brennan et al. is 98.68 g. The study had a 95% confidence interval of 82.36 to 115.0 g, meaning there is a 95% probability that the mechanical threshold for concussion is in that range.

Based on the findings presented by Cripton et al., an e-scooter rider who is thrown from their scooter while not wearing a helmet will likely sustain a head exposure that is approximately 6 to 8.4 times greater than the threshold associated with medically diagnosed concussions. Comparatively, helmeted riders will likely sustain a head exposure that is approximately 1.6 to 1.8 times greater. Stated differently, a helmet will likely prevent serious skull and brain injuries but may not eliminate the risk of a concussion.

While helmets are not required for riders aged 18 or older, when considering the above biomechanical data, helmet use should be encouraged for everyone!

[1] Source: https://ottawa.ctvnews.ca/electric-scooters-roll-out-in-ottawa-1.5975370
[2] Cicchino, J.B., Kulie, P.E., McCarthy, M.L., 2021a. Injuries related to electric scooter and bicycle use in a Washington, DC, emergency department. Traffic Injury Prevention, 22 (5), 401–406.
[3] Blomberg, S.N.F., Rosenkrantz, O.C.M., Lippert, F., Collatz Christensen, H., 2019. Injury from electric scooters in Copenhagen: a retrospective cohort study. BMJ Open 9 (12), e033988.
[4] Posirisuk, P., Baker, C. and Ghajari, M. (2022). Computational prediction of head-ground impact kinematics in e-scooter falls. Accident Analysis and Prevention, 167: 106567.
[5] Source: https://swiftyscooters.com/blogs/journal/scooter-safety-pothole-test-results
[6] Recommended Maintenance Quality Standards for Roads and Sidewalks/Pathways. City of Ottawa. ACS2004-TUP-SOP-0012
[7] Mertz, H.J., & Irwin, A.I. (2003). Biomechanical and scaling bases for frontal and side impact injury assessment reference values. Stapp Car Crash Journal, 47, 155-188.
[8] Cripton, P.A., Dressler, D.M., Suart, C.A., Dennison, C.R., and Richards, D. (2014). Bicycle helmets are highly effective at preventing head injury during head impact: Head-form accelerations and injury criteria for helmeted and unhelmeted impacts. Accident Analysis and Prevention, 70: 1-7.
[9] Mertz, H. J., Prasad, P., & Nusholtz, G. (1996). Head injury risk assessment for forehead impacts (Report No. 960099). Society of Automotive Engineers.
[10] Cripton, P.A., Dressler, D.M., Suart, C.A., Dennison, C.R., and Richards, D. (2014). Bicycle helmets are highly effective at preventing head injury during head impact: Head-form accelerations and injury criteria for helmeted and unhelmeted impacts. Accident Analysis and Prevention, 70: 1-7.
[11] Brennan, J.H., Mitra, B., Synnot, A., McKenzie, J., Willmott, C., McIntosh, A. S., Maller, J.J., & Rosenfeld, J.V. (2017). Accelerometers for the assessment of concussion in male athletes: A systemic review and meta-analysis. Sports Medicine, 47(3), 469-478.