Lane splitting refers to a two-wheeled vehicle moving between roadway lanes of vehicles that are proceeding in the same direction. More narrowly, it refers to overtaking slow or stopped vehicles by traveling between lanes.
It is also sometimes called lane sharing, whitelining, filtering, or stripe-riding. Alternatively, lane splitting has been used to describe moving through traffic that is in motion while filtering is used to describe moving through traffic that is stopped.
Lane splitting by motorcycles is illegal in certain places, such as most US states with the sole exception being California, but widely used and legal in many other countries. Additionally, the legality of lane splitting for motorcycles and bicycles is the same in some places, such as California, but different in other places, such as Nebraska, where lane splitting is prohibited specifically for motorcycles and is therefore lawful for bicycles.
In the developing world
In population-dense and traffic-congested urban areas, particularly in the developing world, the space between larger vehicles is filled with a wide variety of different kinds of two-wheeled vehicles, as well as pedestrians, and many other human or animal powered conveyances. In places such as Bangkok, Thailand and in Indonesia, the ability of motorcycles to take advantage of the space between cars has led to the growth of a motorcycle taxi industry. In Indonesia, the motorcycle is the most common type of vehicle.
Unlike typical developed nations that have only a handful of vehicle types on their roads, many types of transport will share the same roads as cars and trucks; this diversity is extreme in Delhi, India, where more than 40 modes of transportation regularly use the roads. In contrast, New York City, for example, has perhaps five modes, and in parts of America a vast majority of traffic is made up of two types of vehicles on the road, cars and trucks.
It has been suggested that highly diverse and adaptive modes of road use are capable of moving very large numbers of people in a given space compared with cars and trucks remaining within the bounds of marked lanes. On roads where modes of transportation are mingled this can cause a reduced efficiency for all modes.
Filtering forward, or filtering, is a technique used by bicyclists and motorcyclists to pass a stopped or slow-moving lane of congested traffic by traveling in unused lane space. When the space used is between two lines of vehicles, this is also known as lane splitting, but filtering can be accomplished by using space on the outside edge of same-direction traffic as well. There can be a significant saving of time by bypassing what otherwise would be obstructions.
Lane splitting is controversial in the United States, and is sometimes an issue in other countries. Questions are debated as to whether or not it is legal, whether or not it should be legal, and whether or not it should be practiced regardless of legality. Bills to legalize lane splitting have been introduced in state legislatures around the US over the last twenty years but so far none has been enacted.
Filtering forward, in stopped or extremely slow traffic, requires very slow speed and awareness that in a door zone, vehicle doors may unexpectedly open. Also, unexpected vehicle movements such as lane changes may occur with little warning. Buses and tractor-trailers require extreme care, as the cyclist may be nearly invisible to the drivers who may not expect someone to be filtering forward. To avoid a hook collision with a turning vehicle at an intersection after filtering forward to the intersection, cyclists are taught to either take a position directly in front of the stopped lead vehicle, or stay behind the lead vehicle. Cyclists should not stop directly at the passenger side of the lead vehicle, that being a blind spot.
Little safety research in the United States has directly examined the question of lane splitting. Preliminary findings from a University of California, Berkeley study were published in October, 2014 with a full report expected by the end of the year. The European MAIDS report studied the causes of motorcycle accidents in four countries where it is legal and one where it is not, yet reached no conclusion as to whether it contributed to or prevented accidents.
Proponents of lane splitting state the Hurt Report of 1981 reached the conclusion that lane splitting improves motorcycle safety by reducing rear end crashes. Lane splitting supporters also state that the US DOT FARS database shows that fatalities from rear end collisions into motorcycles are 30% lower in California than in Florida or Texas, states with similar riding seasons and populations but which do not lane split. No specifics are given about where this conclusion is found in the FARS system. The database is available online to the public. The NHSTA does say, based on the Hurt Report, that lane splitting "slightly reduces" rear-end accidents, and is worthy of further study due to the possible congestion reduction benefits.
Lane splitting is never mentioned anywhere in the Hurt Report, and all of the data was collected in California, so no comparison was made between of lane splitting vs. non-lane splitting. The Hurt Report ends with a list of 55 specific findings, such as "Fuel system leaks and spills are present in 62% of the motorcycle accidents in the post-crash phase. This represents an undue hazard for fire." None of these findings mentions lane splitting, or rear end collisions. The legislative and law enforcement advice that follows this list does not mention lane splitting or suggest laws be changed with regard to lane splitting.
In Europe, the MAIDS Report was conducted using Organisation for Economic Co-operation and Development (OECD) standards in 1999–2000 and collected data on over 900 motorcycle accidents in five countries, along with non-accident exposure data (control cases) to measure the contribution of different factors to accidents, in the same way as the Hurt Report. Four of the five countries where data was collected allow lane splitting, while one does not, yet none of the conclusions contained in the MAIDS Final Report note any difference in rear-end accidents or accidents during lane splitting. It is notable that the pre-crash motion of the motorcycle or scooter was lane-splitting in only 0.4% of cases, in contrast to the more common accident situations such as "Moving in a straight line, constant speed" 49.1% and "Negotiating a bend, constant speed" 12.1%. The motorcyclist was stopped in traffic prior to 2.8% of the accidents.
Preliminary results from a study in the United Kingdom, conducted by the University of Nottingham for the Department for Transport, show that filtering is responsible for around 5% of motorcycle Killed or Seriously Injured (KSI) accidents. It also found that in these KSI cases the motorist is twice as likely to be at fault as the motorcyclist due to motorists "failing to take into account possible motorcycle riding strategies in heavy traffic".
Debate over safety and benefits
Proponents state that the practice relieves congestion by removing commuters from cars and gets them to use the unused roadway space between the cars, and that lane splitting also improves fuel efficiency and motorcyclists' comfort in extreme weather. In the US, transportation engineers have suggested that motorcycles are too few, and will remain too few, to justify any special accommodation or legislative consideration, such as lane splitting. Unless it becomes likely that very large number of Americans will switch to motorcycles, they will offer no measurable congestion relief even with lane splitting. Rather, laws and infrastructure should merely incorporate motorcycles into normal traffic with minimal disruption and risk to riders.
Potentially, lane splitting can lead to road rage on the part of drivers, who feel frustrated that the motorcyclists are able to filter through the traffic jam. However, the Hurt Report indicates that, "Deliberate hostile action by a motorist against a motorcycle rider is a rare accident cause." Lane splitting is not recommended for beginning motorcyclists, and riders who do not practice it in their home area are strongly cautioned that it can be risky if they attempt it when traveling to a jurisdiction where it is allowed. Similarly, for drivers new to places where it is done, it can be startling and scary.
Responsibility and liability issues
Another consideration is that lane splitting in the United States, even where legal, can possibly leave the rider legally responsible. 'Safely' is always very much a judgment call. The mere fact that an accident happened while a rider was lane splitting is very strong evidence that on that occasion it wasn't safe to do so...If you've been involved in an accident you will have a hard job convincing an insurance adjuster that the accident was not completely your fault."
When the 2005 bill to legalize lane splitting in Washington State was defeated, a Washington State Patrol spokesman testified in opposition, saying that, "it would be difficult to set and enforce standards for appropriate speeds and conditions for lane splitting. And he said that officials with the California Highway Patrol told him that they wished they had never begun allowing the practice. Recently, California Highway Patrol removed their guildelines on such practice. 
California's DMV handbook for motorcycles advises significant caution regarding lane splitting: "Cars and motorcycles each need a full lane to operate safely. Lane sharing is not safe. Riding between rows of stopped or moving cars in the same lane can leave you vulnerable. A car could turn suddenly or change lanes, a door could open, or a hand could come out of a window. Discourage lane sharing by others." The Oxford Systematics report commissioned by VicRoads, the traffic regulating authority in Victoria, Australia, found that for motorcycles filtering through stationary traffic "No examples have yet been located where such filtering has been the cause of an incident."
In the United Kingdom, Motorcycle ROADCRAFT, the police riding manual, is explicit about the advantages of filtering but also states that "The advantages of filtering along or between stopped or slow moving traffic have to be weighed against the disadvantages of increased vulnerability while filtering".
After discussing the pros and cons at great length, motorcycle safety guru David L. Hough ultimately argues that a rider, given the choice to legally lane split, is probably safer doing so, than to remain stationary in a traffic jam. However, Hough has not gone on record as favoring changing the law in jurisdictions where it is not permitted, in contrast to his public education and legislative efforts in favor of rider training courses and helmet use. A literature review of lane-sharing by the Oregon Department of Transportation notes "a potential safety benefit is increased visibility for the motorcyclist. Splitting lanes allows the motorcyclist to see what the traffic is doing ahead and be able to proactively maneuver." However, the review was limited and "Benefits were often cited in motorcyclist advocacy publications and enthusiast articles."
A frequently asked question by motorcyclists is "Is lane splitting legal?" In Australia a furor erupted when the transport authorities decided to consolidate and clarify the disparate set of laws that collectively made lane splitting illegal. Because of the very opacity of the laws they were attempting to clarify, many Australians had actually believed that lane splitting was legal, and they had been practicing it as long as they had been riding. They interpreted the action as a move to change the law to make lane splitting illegal. Because of the volume of public comment opposed to this, the authorities decided to take no further action and so the situation remained as it was until 1 July 2014 when New South Wales made lane filtering and lane splitting legal under strict conditions. On 1 February 2015, similar relaxations were introduced in Queensland.
The legal confusion in Australia is not exceptional. In a 2012 California survey only 53% of non-motorcycle drivers thought lane splitting is legal. California is not the only state in which there is no traffic law that explicitly prohibits lane splitting, but officials rely on other laws to regularly interpret lane splitting as unlawful. For example, New Mexico does not address lane splitting by name, but has language requiring turn signals be used continuously for at least 100 ft (30 m) before changing lanes, as well as other codes which may be cited by an officer. Many other states have identical codes, derived from the Uniform Vehicle Code.
Other jurisdictions have similar or identical current legal codes, yet their authorities have, over time, interpreted the law as prohibiting lane splitting in all cases, even when done safely, and so riders are cited for it. For instance, in Colorado and Nebraska the law explicitly prohibits lane splitting, while permitting motorcycles to ride two abreast, and making an exception for police officers.
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- "It's often safer to take the whole lane, or at least ride a little bit to the left, rather than hug the right curb. Here's why: Cars at intersections ahead of you can see you better if you're squarely in the road rather than on the extreme edge where you're easily overlooked. ..." Michael Bluejay, bicyclesafe.com
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The OTS survey showed that only 53 percent of vehicle drivers thought that lane splitting is legal in California.
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- Besides violating 66-7-325 Turning Movements and required signals prohibit Lane Splitting, a motorcyclist lane splitting in New Mexico could be cited for 66-7-317 "Driving on roadways laned for traffic"  and 66-7-322 "Required position and method of turning at intersections" 
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All available from the United Kingdom Department of Transport websites (executive summary), and the Transportation Research Board Record publication:
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- WSP Policy and Research UK, Motorcycles and congestion: the effect of modal shift: Phase 2 - Modelling Methodology. 2004, WSP for Uk Department for Transport: Cambridge UK. p. 47.
- WSP Policy and Research UK, et al., Motorcycles and congestion: the effect of modal shift: Summary Final Report. 2004, WSP for Uk Department for Transport: Cambridge UK. p. 26.
- Burge, P., et al., The modelling of motorcycle ownership and usage: a UK study. Transportation Research Record J Transportation Research Board, 2007(2031): p. 59-68.