Lane-sharing is the practice by a motorcyclist of sharing a lane with another vehicle. Lane-splitting is the practice by a motorcyclist of riding between streams of traffic by riding on or near the striping between lanes. Filtering, which seems to be a term frequently used outside of the United States, is the practice by a motorcyclist of riding between lines of stopped or nearly stopped traffic at a traffic signal. The terms lane-splitting and lane-sharing are often used interchangeably and often the term lane-sharing is used as a catch-all term for all of these practices. These practices, which are allowed in California, allow a motorcyclist to drive faster than the surrounding traffic in congested or stop-and-go traffic [1, 2]. They also give motorcyclists more options for how they position themselves on the roadway, allowing them to “strategically place themselves in pockets of lower congestion during commute traffic” and to “distance themselves from safety hazards from larger vehicles beside or behind them, or from hazards presented by highly congested clusters of traffic” .
Guderian  makes similar observations, noting that lane-sharing “is actually a viable safety technique that removes the motorcycle and rider from the danger spot behind a stopped car, and places the motorcycle into the more secure safety envelope that is created between two larger vehicles.” And, in fact, crash statistics do show that lane-sharing riders are less likely to be rear-ended . Guderian reported that “California [where lane-sharing is legal] does have a lower rate of fatal rear-end motorcycle crashes when compared to other similar motorcycling states [where lane-sharing is not legal].” On the other hand, lane-splitting riders are more likely to rear-end another vehicle and “lane-splitting riders often put themselves closer to other vehicles than they otherwise would. This proximity reduces the time riders have to identify and react to changes in the behaviors of other motorists. Anecdotal evidence suggests that the primary risk while lane-splitting is the lane-changing of other vehicles. Other drivers often initiate lane changes without first checking for lane-splitting motorcycles” .
The distance between cars in adjacent lanes is likely to be in the neighborhood of 4 to 6 feet, assuming the cars are traveling side-by-side. This range assume lane widths that are approximately 12 feet and car widths between 6 and 8 feet. If a motorcyclist was riding in the middle of this space between two cars, the centerline of the motorcycle would be approximately 2 to 3 feet from the adjacent cars. If the motorcycle width was around 2.5 feet at the outer extent of the handlebars, then a lane-splitting motorcyclist could come within 0.75 to 1.75 feet of an adjacent car. This small distance would only occur if a motorcyclist was passing between two cars traveling side-by-side in adjacent lanes and it would likely be a short-lived situation. In practice, the cars in adjacent lanes are likely to be staggered to some degree and the motorcyclist would have the freedom to move laterally in either direction to maximize their distance from any particular car they are passing.
As a matter of illustration, I used PC-Crash simulation software to simulate four cars making a 12-foot lane change to the right. The PC-Crash lane change model was used, and the severity of the lateral movement was varied between the four cars. The peak lateral accelerations during the first half of each lane change were as follows: 0.15 g, 0.21 g, 0.29 g, and 0.35 g. The lower end of this spectrum would be consistent with a normal lane change and the higher end represents a more aggressive lane change. In the PC-Crash simulation of the lane change with a peak acceleration of 0.15 g, it took the car approximately 1.2 seconds to move laterally by 2 feet. As the peak accelerations increased to 0.21 g, 0.29 g, and 0.35 g, it took the vehicle 1.0 s, 0.9 s, and 0.87 s, respectively, to move 2 feet laterally. Lane change timing can vary depending on the steering inputs a driver chooses to use, but these numbers illustrate that a driver making a lane change in the vicinity of a lane-splitting motorcyclist could enter the path of the motorcycle in a timeframe that could be shorter than the perception-response time the motorcyclist would need in order to act to avoid the encroaching vehicle.
Research by Muttart over the last two decades has done much to advance the way that accident reconstructionists evaluate perception-response times [5, 6, 7, 8]. In the past, it was common for reconstructionists to cite rules of thumb – for instance, to assume that a reasonable perception-response time for a driver was always 1.5 s, regardless of the situation. Muttart’s work has demonstrated that perception-response times are situation dependent and that there is variability in the response times within a population of drivers. These facts were perhaps obvious but not often used by reconstructionists in practice. Muttart’s work has given reconstructionists the ability to apply these ideas to their practice by developing mathematical equations for predicting driver perception-response times for various situations and implementing these equations in his software, I.DRR (Interactive Driver Response Research).
These equations are based on data from more than 200 driver response studies. The following scenarios are addressed within I.DRR: (1) drivers responding to lead vehicles that were either stopped or moving slowly, (2) drivers responding to being cut off, (3) drivers responding to vehicles intruding into their path, and (4) drivers responding to traffic signals. Within each of these categories, Muttart has demonstrated that apparent discrepancies between the results of various studies can be explained in terms of differences in methodology. He noted that, “in general, as the methodology of the experiment became closer to that of real life, the response times increased.” Also, “response times increased from laboratory to simulator to closed course and then to road studies.”
According to I.DRR, the 85-percentile perception-response time for a motorcyclist who was cutoff during the daytime would be approximately 1.2 seconds. If the motorcyclist was adjacent to a passenger car making a lane change, a crash could be unavoidable to the motorcyclist. On the other hand, if the motorcyclist was behind the car when the incursion began, then the motorcyclist’s ability to avoid would be dependent on how far behind the car they initially were and their speed relative to the car. Their perception-response time and braking skill could also come into play. From an accident reconstruction perspective, crashes involving a lane-splitting motorcyclist would have to be evaluated on a case-by-case basis to determine the relative speed between the encroaching car and the motorcycle, the initial distance between the car and the motorcycle, and the timing of the lane change or lateral movement.
At any rate, data from California shows that lane splitting is generally safe within certain speed constraints. Ouellet collected data related to the prevalence of lane-splitting for weekday, rush-hour conditions by monitoring real-time video feeds from California Department of Transportation (Caltrans) cameras . He found that the percentage of motorcyclists who were lane-splitting varied with the traffic speed. When traffic was moving at a speed between 0 and 10 mph, 90% of motorcyclists were lane-splitting. At speeds between 11 and 20 mph, 81% were lane splitting, 70% at speeds between 21 and 30 mph, and 59% at speeds between 31 and 40 mph. The percentages of motorcyclists who were lane-splitting declined significantly at speeds exceeding 40 mph – 20% at speeds between 41 and 50 mph and 11% at speeds above 50 mph. Ouellet also noted that “motorcycles were overwhelmingly likely to be in the two lanes closest to the center divider.” And also, “riders maintaining a normal lane position in heavy freeway traffic were significantly over-represented in crashes while those who were lane splitting were under-represented.”
Rice, Troszak, and Erhardt [2, 4] reported analysis of data from the California Enhanced Motorcycle Collision Data Project. They examined 5,969 traffic collisions involving motorcycles that occurred between June 2012 to August 2013 and found that, in 997 instances, the motorcyclists were lane-splitting at the time of the collision. They report that “motorcyclists who were lane-splitting were notably different from those that were not lane-splitting. Compared with other motorcyclists, lane-splitting motorcyclists were more often riding on weekdays and during commute hours, were using better helmets, and were traveling at lower speeds. Lane-splitting riders were also less likely to have been using alcohol and less likely to have been carrying a passenger.” Further, “lane-splitting motorcyclists were also injured much less frequently during their collisions. Lane splitting riders were less likely to suffer head injury (9% vs 17%), torso injury (19% vs 29%), extremity injury (60% vs 66%), and fatal injury (1.2% vs 3.0%). Lane-splitting motorcyclists were equally likely to suffer neck injury, compared with non-lane-splitting motorcyclists.” Finally, Rice et al reported that “both traffic speed and motorcycle speed differential (the difference between motorcycle speed and traffic speed) were important in predicting the occurrence of injury. There was no meaningful increase in injury incidence until traffic speed exceeded roughly 50 mph. Motorcycle speed differential was a stronger predictor of injury outcomes. Speed differentials up to 15 mph were not associated with changes in injury occurrence; above that point, increases in speed differential were associated with increases in the likelihood of injury of each type. Lane-splitting appears to be a relatively safe motorcycle riding strategy if done in traffic moving at 50 mph or less and if motorcyclists do not exceed the speed of other vehicles by more than 15 mph.”
Lane-splitting has never been prohibited by California law and a law passed in 2016 (Assembly Bill No. 51 and California Vehicle Code 21658.1) effectively made lane-splitting legal by allowing the California Highway Patrol (CHP) to establish lane-splitting guidelines [9, 10]. The text of the law reads that “the Department of the California Highway Patrol may develop educational guidelines relating to lane splitting in a manner that would ensure the safety of the motorcyclist and the drivers and passengers of the surrounding vehicles.” The CHP had issued guidelines for lane-splitting prior to the passage of this law, but they had been retracted because of a complaint. These guidelines had stated that motorcyclists should not lane split at speeds greater than 30 mph and they should not exceed the speed of surrounding traffic by more than 10 mph. The CHP released updated safety guidelines on September 27, 2018 . These guidelines noted that “lane splitting is a privilege enjoyed by California motorcyclists. With this freedom comes a greater responsibility for motorcyclists and drivers to share the road and create a safer highway environment.” Absent from the new guidelines is any specific statements about the speeds at which motorcyclists should lane split.
The CHP offered the following tips for motorcyclists who are lane-splitting:
· Consider the total environment when you are lane splitting (this includes the width of lanes, the size of surrounding vehicles, as well as current roadway, weather, and lighting conditions).
· Danger increases at greater speed differentials.
· Danger increases as overall speed increases.
· It is typically safer to split between the far left lanes than between the other lanes of traffic.
· Try to avoid lane splitting next to large vehicles (big rigs, buses, motorhomes, etc.).
· Riding on the shoulder is illegal; it is not considered lane splitting.
· Be visible – avoid remaining in the blind spots of other vehicles or lingering between vehicles.
· Help drivers see you by wearing brightly colored/reflective protective gear and using high beams during daylight hours.
For other motorists, the CHP stated, “Motorists can also do their part by sharing the road. Many motorcycle collisions are caused when other motorists simply do not see the motorcyclist. Check your mirrors and blind spots frequently, especially before changing lanes or making a turn. Be alert and courteous while sharing the road.” Non-motorcyclists often have a negative impression of lane-splitting, lane-sharing, and filtering. Beanland, Pammer, Sledziowska, and Stone  conducted a survey of 249 drivers, just prior to the legalization of filtering in New South Wales and Queensland in February 2015. They reported that “most drivers (61%) reported witnessing lane filtering at least once per week. Many drivers (28%) mistakenly believed lane filtering was already legal…but 70% stated it should be illegal. Drivers were significantly more likely to agree lane filtering should be legal if they believed it was already legal… [or] rode a [motorcycle] themselves…Reasons for endorsing lane filtering varied but included: easing traffic congestion; improving safety; personal freedom; and difficulty enforcing lane filtering prohibitions. Reasons for opposing lane filtering primarily related to safety concerns and drivers’ difficulties in perceiving motorcycles (e.g., in blind spots).”
These authors also reported that “a minority of drivers appear to oppose lane filtering on ideological grounds. Specifically, a number of drivers believed that laws should be consistent between different classes of road users to maintain consistency and fairness. This includes the idea that [motorcycle] riders should just be patient and wait their turn. Although this appears to be a minority opinion, in jurisdictions where lane filtering is (or may become) legal, it would be worthwhile attempting to identify ways in which lane filtering could benefit drivers in order to minimize the potential for aggressive and hostile interactions between road users.”
Ewald & Wasserman  surveyed 709 motorcyclists and 951 passenger vehicle drivers in California. The motorcyclists were asked if they had hit or been hit by a vehicle in the previous 12 months. Approximately 93% of motorcyclists indicated they had not hit or been hit by a vehicle. Approximately 5% reported they had been hit by a car and approximately 2% reported they had hit a car. Of the 93% of motorcyclists who had not been involved in a collision with a car, approximately 21% stated that they had nearly been involved in a collision with a car. Of the motorcyclists who had been involved in a collision with a car, approximately 28% struck a car’s side mirror. Approximately 23% of the motorcyclists involved in a collision suffered minor injuries and 10% suffered severe injuries. Approximately 54% of lane-splitting motorcyclists reported that during the previous 12 months, a passenger vehicle driver had tried to prevent them from passing. Approximately 87% of motorcyclists reported lane-sharing only when traffic speeds were 40 mph or less. Approximately 93% of motorcyclists reported keeping their speed within 15 mph of the speed of other traffic when lane-sharing.
Of the passenger vehicle drivers who were surveyed, approximately 61% thought it was legal to lane-split in California, 29% percent thought it was illegal, and 10% did not know. Approximately 34% of vehicle driver strongly disapproved of lane-splitting, 27% somewhat disapproved, 29% somewhat approved, and 10% strongly approved. The most common reason for disapproval of lane-splitting was “it is unsafe.” Approximately 4% of vehicle drivers indicated that they had tried to prevent a lane-splitting motorcyclist from passing them in the prior 12 months. Approximately 97% of vehicle drivers reported that they had not hit or been hit by a motorcyclist in the previous 12 months, whereas 3% had hit or been hit by a motorcyclist. Of the vehicle drivers whose cars had been contacted by a motorcyclist, 48% reported that the motorcyclist just contacted their side mirror, 24% reported that the motorcycle scraped or hit the side of their car, and 8% reported that they had knocked the motorcyclist down. Approximately 27% of passenger vehicle drivers reported nearly being hit by a lane-splitting motorcyclist.
Ouellet, James V., “Lane Splitting on California Freeways,” Motorcycle Accident Analysis.
Rice, T., Troszak, L., Erhardt, T., “Motorcycle Lane-Splitting and Safety in California,” Safe Transportation Research & Education Center, University of California Berkeley, May 29, 2015.
Guderian, Steve, “Lane Sharing: A Global Solution for Motorcycle Safety,” August 2011.
Rice, T., Troszak, L., “Safety Implications of Lane-Splitting Among California Motorcyclists Involved in Collisions,” Safe Transportation Research & Education Center, University of California Berkeley, August 6, 2014.
Muttart, J.W., “Estimating Driver Response Times,” Chapter 14 of Handbook of Human Factors in Litigation, CRC Press, 2005.
Muttart, J.W., “Evaluation of the Influence of Several Variables Upon Driver Perception Response Times,” Proceedings of the 5th International Conference of the Institute of Traffic Accident Investigators, York, England, 2001.
Muttart, J., "Development and Evaluation of Driver Response Time Predictors Based upon Meta-Analysis," SAE Technical Paper 2003-01-0885, 2003, doi:10.4271/2003-01-0885.
Muttart, J.W., “Quantifying Driver Response Times Based Upon Research and Real Life Data,” Proceedings of the Third International Driving Symposium on Human Factors in Driver Assessment, Training and Vehicle Design, June 2005.
Levi, Ryan, “10 Things to Know About Lane Splitting in California (No. 1? Yes, It’s Legal.)” KQED News, https://www.kqed.org/news/11680481/10-things-you-need-to-know-to-understand-lane-splitting-in-california, published on July 19, 2018, accessed on February 9, 2019.
Press Release, “CHP Announces Lane Splitting Tips,” https://www.documentcloud.org/documents/4946461-Lane-Splitting-Press-Release.html, accessed on February 9, 2019.
Beanland, V., Pammer, K., Sledziowska, M., Stone, A., “Drivers’ attitudes and knowledge regarding motorcycle lane filtering practices,” Proceedings of the 2015 Australasian Road Safety Conference, October 14-16, Gold Coast, Australia.
Ewald & Wasserman: Research Consultants, LLC, “Motorcycle Lane-Share Study Among California Motorcyclists and Drivers 2014 and Comparison to 2012 and 2013 Data: Methodological and Analysis Report,” May 2014.