The Following are guidelines the St. Croix county Highway Department follows with respect to the installation and ongoing maintenance of mailboxes on the county trunk highway system (CTHS).
In Wisconsin snow removal operations frequently conflict with mailboxes. A standard for mailbox installation has been developed which has reduced the number of mailboxes damaged or demolished by snow plows.
The recommended placement of the box is on a 24 inch minimum arm length at a minimum height of 46 inches. The front of the box shold be aligned with the outside edge of the shoulder.
Locating mailboxes along the roadside is complicated by many factors other than snow removal. These require balancing the maximum convenience to the patron with safety considerations for highway traffic, and with postal carrier convenience and safety. You must also take into account existing roadside conditions such as proximity to intersection: blind spots caused by curves, hills and vegetation: utility poles, guardrails, nearby bridges, and other physical installations.
GUIDELINES FOR SAFE INSTALLATION
It is best to keep the entire mailbox installation as light weight as possible. This permits a vehicle crashing into it to break off the mailbox with less damage to the vehicle. The heavier the installation, the less likely it will move instantaneously, causing more damage to the vehicle and injury to its occupants.
The postal service requires that the bottom of the box be 42 to 48 inches above ground level. the Wisconsin Department of Transportation has these standards for supports which will ensure that they will break away on impact: