These banks of mailboxes are often used in new construction developments in place of letter openings in the property doors. In 2019 the UK government agreed to ban low-level letter boxes after a campaign from the Royal Mail. United States Numerous designs of mailboxes with improved construction and security have been patented in recent years, particularly in the United States. In 2001, the USPS first approved designs for locking curbside mailboxes to stem a rise in identity and mail theft. With these secure designs, the incoming mail is placed into a slot or hopper by the mail carrier, where it drops into a secure locked compartment for retrieval by only the homeowner (who retains a key or combination to the lock). Locking mailboxes are generally constructed of heavy-gauge steel or aluminum plate, though some models are made of roto-moulded polymer plastic. Because of the increased risk of vandalism to curbside mailboxes, numerous vandal-resistant boxes made of composite plastic or heavy-gauge steel or aluminum plate have also entered the market. Some composite mailboxes made of resilient polymer plastics and mounted on ground spikes can resist severe impacts from baseball bats or even being run over by a vehicle. In 1978, steady increases in postal service costs caused the USPS to insist on either curbside or centralized mail delivery for new suburban neighborhoods and developments. A 1995 cost delivery study published in a USPS Operations handbook listed per-address annual delivery costs as: Door-to-door, $243; Curbside, $154; Cluster/flock Box (centralized mail delivery), $106. Large apartment buildings usually have a cluster/flock of mailboxes for all units, located in the entry lobby or in a nearby dedicated mailroom. There often is a special lock box (also called a "key keeper" or "Knox box") located at the outside entrance, which either gives access to a front door key or directly activates the front door electric strike, to allow the mail delivery person to enter the building. A similar "Arrow lock" is usually located on the centralized mailbox, to allow the entire unit to be opened for efficient filling of individual mailboxes. In the US, a property with a single mailing address but with multiple mail recipients may utilize a community mail station designated CBU, or Cluster Box Unit. CBUs are typically stand-alone units that have locked individual compartments for each tenant in an apartment building, a trailer or mobile home park, or an office center. By