authorities to develop forward looking policy documents such as Local Plans or Unitary Development Plans to outline what kind of development is permitted where, and to mark special areas on Local Plan Maps (today referred to as policies map). It did not introduce a formal system of zoning as used in the United States. Counties developed Structure Plans that set broad targets for the wider area. Structure Plans were always problematic and were often in the process of being replaced by the time they were formally adopted. Over the years, the planning system has undergone a number of alterations, which were consolidated in the Town and Country Planning Act 1990 (TCPA 1990). Section 106 substantially re-wrote Section 52 from the former Act, settling the concept of agreements (known as "planning obligation agreements," or more commonly "Section 106 agreements"), under which the developer is subject to detailed arrangements and restrictions beyond those that a planning condition could impose, or by which they make agreed financial contributions beyond the immediate building works to offset development effects on the local community. This was soon amended to allow a developer to self-impose obligations to preempt objections to planning permission. This prevents the planning authority from blocking a permission by merely failing to negotiate. Three further Acts related to planning are associated with this primary act: The Planning (Listed Buildings and Conservation Areas) Act 1990, the Planning (Hazardous Substances) Act 1990, and the Planning (Consequential Provisions) Act 1990. These four Acts are referred to as the Planning Acts. Almost immediately after parliament passed these Acts, the government had further thoughts on the control of land development, which led to the Planning and Compensation Act 1991, which made important alterations to many of the Planning Acts provisions. The Planning and Compulsory Purchase Act 2004 made substantial changes to the English Development Plan system. It did away with both Structure Plans and Local Plans, in favour of Local Development Frameworks (LDFs), which are made up a number of Local Development Documents (LDDs) and Supplementary Planning Documents (SPDs). The Regional Spatial Strategy (RSS), which is produced by Regional Assemblies in England, replaces the Structure Plan as the strategic planning document (i.e., the RSS that's targets for housing and employment development within each district in a Region in the future). A variation on this approach exists in Wales. Local Authorities are also now required to produce Local Development Schemes