Olmsted is well-known for co-creating several well-known urban parks with his senior collaborator, Calvert Vaux. One of Olmsted's early efforts was the design of New Britain, Connecticut's Walnut Hill Park. His latter projects included New York City's Central Park and Prospect Park, as well as Trenton's Cadwalader Park. In addition to being a renowned landscape architect, Olmsted was also an advocate for natural history teaching in schools and served as president of the National Academy of Sciences. His work contributed to the advancement of urban life by making cities more pleasant and accessible. Before the advent of motor vehicles, city streets were not always designed for ease of travel. Traffic could be heavy, especially during morning and evening rush hours, and this made walking around cities difficult. The introduction of traffic lights in 1898 helped make cities safer by reducing accidents caused by poor street lighting. The creation of green spaces in cities has been important for mental health. Residents who live near parks are less likely to suffer from anxiety or depression. This may be because spending time in nature helps release endorphins into the brain, which in turn makes people feel happier. Cities without green space become increasingly crowded and expensive. Land prices rise because developers seek out undeveloped land to build on, and residents need places to go for exercise and nature exposure.