An earplug is provided and includes a tip for use in forward and lateral sealing, which is formed to define a rearward recessed cavity, and a stem. A packet sent from a sending node to a destination node is received at a monitoring node. The indication from the measuring device may be adapted to provide a quantifiable indication to a driver or crew related to recent changes in the tractive condition of the surface. when having a detachable upper portion, the sleeve has a detaching element which, when employed to detach the upper portion, leaves a linear or non-linear upper edge on the lower portion of the sleeve. The fail-safe circuitry may also include an energy-transfer device coupled between the first and second solenoid-controlled valves. The transport wheels are pivotable between a transport position and an operating position, and are located further outward in the viewpoint of the vibration plate in the width direction in the transport position than in the state of the operating position.