def vroll(vel_1, vel_2):
    """

    Calculate the rolling speed in a tribological contact based on contact body
    velocities.

    Parameters
    ----------
    vel_1: ndarray, scalar
        The contact velocity of body 1.
    vel_2: ndarray, scalar
        The contact velocity of body 2

    Returns
    -------
    vel_roll: ndarray, scalar
        The rolling velocity in the tribological contact.

    """
    vel_roll = (vel_1 + vel_2) / 2
    return vel_roll
