def parse_max_acc(xmldoc):
    """parses the vehicle acceleration distributions from the VISSIM data
    :param xmldoc:  input VISSIM xml
    :type xmldoc:   xml.dom.minidom.Document
    :return:    map of 1st acceleration function data point value by str(numeric id)
    :rtype:     dict
    """
    acc_d = dict()
    for max_acc in xmldoc.getElementsByTagName('maxAccelerationFunction'):
        acc_d[max_acc.getAttribute('no')] = max_acc.getElementsByTagName(
            'accelerationFunctionDataPoint')[0].getAttribute('y')
    return acc_d