However, it was only in the early 20th century, as the street lost its role as the default public space and became reserved for use by motor cars, that momentum built to remove children from the new dangers and confine them to segregated areas to play. In the United States, organisations such as the National Highway Protective Society highlighted the numbers killed by automobiles, and urged the creation of playgrounds, aiming to free streets for vehicles rather than children's play. The Outdoor Recreation League provided funds to erect playgrounds on parkland, especially following the 1901 publication of a report on numbers of children being run down by cars in New York City.
In tandem with the new concern about the danger of roads, educational theories of play, including by Herbert Spencer and John Dewey inspired the emergence of the reformist playground movement, which argued that playgrounds had educational value, improved attention in class, enhanced physical health, and reduced truancy. Interventionist programs such as by the child savers sought to move children into controlled areas to limit 'delinquency'. Meanwhile, at schools and settlement houses for poorer children with limited access to education, health services and daycare, playgrounds were included to support these institutions' goal of keeping children safe and out of trouble.
One of the first playgrounds in the United States was built in San Francisco's Golden Gate Park in 1887. In 1906 the Playground Association of America was founded and a year later Luther Gulick became president. It later became the National Recreation Association and then the National Recreation and Park Association. Urging the need for playgrounds, former President Theodore Roosevelt stated in 1907:
City streets are unsatisfactory playgrounds for children because of the danger, because most good games are against the law, because they are too hot in summer, and because in crowded sections of the city they are apt to be schools of crime. Neither do small back yards nor ornamental grass plots meet the needs of any but the very small children. Older children who would play vigorous games must have places especially set aside for them; and, since play is a fundamental need, playgrounds should be provided for every child as much as schools. This means that they must be distributed over the cities in such a way as to be within walking distance of every boy and girl, as most children can not afford to pay carfare.
In post war London the landscape architect and children’s rights campaigner Lady Allen of Hurtwood introduced and popularised the concept of the ’junk playground’ - where the equipment was constructed from the recycled junk and rubble left over from the Blitz. She campaigned for facilities for children growing up in the new high-rise developments in Britain's cities and wrote a series of illustrated books on the subject of playgrounds, and at least one book on adventure playgrounds, spaces for free creativity by children, which helped the idea spread worldwide.