A protected intersection is an at-grade road junction in which cyclists and pedestrians are separated from cars. Vehicles turning right (in countries driving on the right, or left in countries driving on the left) are separated from crossing cyclists and pedestrians by a buffer, providing increased reaction times and visibility. Drivers looking to turn right have better visibility of cyclists and pedestrians as they can look to the side for conflicts instead of over their shoulders. At unsignalized intersections, it is practice to have one car length of space between the cycleway and roadway, so that cars exiting the minor street have an area to pull forward and wait for a gap in traffic, without becoming distracted by potential simultaneous conflicts along the cyclepath.
This type of intersection has for decades been used in the bike-friendly Netherlands, and Denmark. It is also one of the main ways cyclists are transported across intersections in central European countries, and in outlying suburban areas in China. An alternative philosophy, design for vehicular cycling, encourages having bike lanes simply disappear, or "drop", at intersections, forcing riders to merge into traffic like a vehicle operator ahead of the intersection in order to avoid the risk of a right-hook collision, when a right turning motorist collides with a through moving cyclist. Design policies which do not allow the cyclist to remain separated through the intersection have come under increasing scrutiny in recent years as not being inclusive to riders of all abilities, leading to lower overall ridership and sidewalk riding, and being less safe.
Other countries and jurisdictions where they were not previously common are beginning to install, and/or incorporate into standard details, protected intersections with varying degrees of similarity to those in the Netherlands, including the UK city of Manchester,  Australian city of Melbourne,  over two dozen U.S. locations, including in Austin, Boston, Chicago, Davis, Pittsburgh, Salt Lake City, San Diego, Seattle, Silver Spring, Jersey City, and Canadian cities Ottawa, Vancouver, and Waterloo.
With the popularity of the bicycle, the Dutch began constructing separated cycle tracks as early as the late 1800s. The separation of road users into neatly defined right-of-ways may be linked to Dutch culture in general, which values cleanness and organization. After World War II, the country's infrastructure was decimated, and some cities like Rotterdam had to be completely rebuilt. This presented the opportunity to create infrastructure more in line with the "modern" way. From the 1940s to the 70s, streets were built following a new design philosophy that attempted to integrate cyclists with vehicle traffic. After three decades, these designs proved to be largely a failure, with the number of kilometers cycled falling by 65% and the per-km rate of cyclists being killed increasing 174%.
In the 1970s, road traffic and urban quality of life began to be seen as a significant issue in Dutch city politics. This, combined with other political headwinds related to party reorganization, the decline of national religious pillars, and opposition to the Vietnam War propelled left wing political parties to office in many city governments. In Groningen, a northern Dutch city with one of the highest bike mode shares, the left wing party put forth a new circulation plan which again prioritized bicycle traffic and moved away from the notion of designing for bicyclists to act like motor vehicle operators. As the nation again began to desire separated bike infrastructure, the protected intersection rose to prominence as an engineering solution for optimizing sightlines. It joined other Dutch innovations in traffic calming and bike design, like the woonerf, and the bicycle street (fietsstraat), a variant of which exists in North America (see bicycle boulevard). Today, the Netherlands is widely considered the world's premier country for cycling, with more than 25% of all trips made by bike. It has reported a significantly lower cyclist fatality rate following the return to separated infrastructure. In the US, 58% of bicycle crashes involving injury, and 40% of crashes involving death occurred at intersections.
The protected intersection is only one of several treatments for addressing motorist-cyclist conflicts. While used in much of the Netherlands, including Amsterdam, local road authorities in other parts of the country do not use the classic protected intersection with middle islands, preferring to have cyclists move during a completely separated all directions green phase. Other options for reducing bike accidents at intersections, depending on context, include the use of bridges and tunnels, and planning or reconfiguring the neighborhood street/path system so that major amenities and schools can be reached without needing to travel along busy roads.
This protected intersection example features a number of common elements that optimize safety.
Clear ground striping is key to define the cycle lane and its priority. Wide strips are painted aside the cycle way and 'shark teeth' (triangles with pointy end oriented toward the non priority vehicles) are used to reinforce who must yield. In addition to ground marking, the cycle lane color plays a role to remind motorists of cyclist priority. In the Netherlands, the cycle way red color is not painted but embedded in asphalt to increase durability and reduce costs.
Also often the cycle lane is slightly raised in the crossing, to again invite motorists to decrease speed.
Cyclists ideally have a protected bike lane on the approach to the intersection, separated by a concrete median with splay curbs if possible, and have a protected bike lane width of at least 2 meters if possible (one way). In the Netherlands, most one way cycle paths are at least 2.5 meters wide.
Reduced radius could increase difficulties to turn for larger vehicles (trucks and busses), so in some cases, mountable islands have been used, similarly to the truck mountable aprons which surround the center island of roundabouts.
In terms of optimal spacing between the path and motorist lanes, it is generally practice to use 2-5 meters at signalized crossings and one car length >5m at unsignalized intersections. Providing more buffer space allows vehicles, particularly those turning out of smaller roads, to queue in the waiting area. On the other hand, larger buffers could place the cyclist at a less optimal viewing point from the mainline, and delay the signal operation due to longer distances necessitating slightly longer bicycle signal yellow and all red clearance intervals. The exact optimal distance has been the subject of several studies.  
Protected roundabouts are a variation of protected intersections for lower traffic flow, without the traffic lights.
Alternative design with increased distance from intersection for cycle and pedestrian crossing and motorists having priority over cyclists may be safer and more practical with double direction cycle path.
In The Netherlands, a not for profit organisation working on standardisation and research on traffic, transport and infrastructure, the CROW (dutch), have published since 2006 a design manual for all cycling infrastructure, with an English version of the last edition of 2017.
The concept of Dutch protected intersection got international public attention after an April 2011 blog post ‘State of art bikeway design, or is it ?’ by Mark Wagenbuur  on David Hembrow blog associated with a video  criticizing a US junction design published in NACTO design guide, his post trying to explain the Dutch philosophy of pedestrian and cyclist friendly junctions. To emphasize his point, the video schematics were based upon dimensions and schematics of the NACTO publication.
A month later, Mark Wagenbuur published another post trying to clarify the concept and avoid misunderstanding: ‘State of the art bike way design - a further look ’  with a new video.
In the USA, Nick Falbo, a member of a US planning firm which was originating the NACTO design, Alta planning+design, published in February 2014 a small web presentation site ‘protectedintersection.com’  of his interpretation of Mark Wagenbuur posts.
In February 2014, David Hembrow published a blog post «The myth of standard Dutch junction» 
The same month, Mark Wagenbuur published on his own blog a new clarification post ‘Junction design in the Netherlands’ criticising some design he considers being a wrong interpretation of his posts.
Then Alta planning+design published in December 2015 a PDF presentation with a short USA history, schematics and dimensions and some examples of realisations of ‘protected intersections’ in the USA and Canada which may differ from the Dutch practice.
In 2015, the Massachusetts DOT released their Separated Bike Lane Planning and Design Guide, which includes extensive discussion of protected intersections, and was used as a pilot for the upcoming AASHTO Bike Guide, which is expected to include protected intersections as well.
In 2019, NACTO, whose original Urban Bikeways Design guide generated controversy for omitting the treatment, released Don't Give Up at the Intersection, which encourages protected intersections as an alternative to bike lane drops. They also released Designing for All Abilities, which again encourages separated treatments along major roads and intersections.