var WIDTH = 307;
var left = 0;

function StartCreep(objectName, interval, step) {
    var line = document.getElementById(objectName);
    //todo duplicate content
    setInterval(function () { Creep(line, step); }, interval);
}

function Creep(line, step) {
    left = left - step;

    if (left > WIDTH) {
        left = -WIDTH;
    }

    if (left < -WIDTH) {
        left = -5 - step;
    }

    line.style.left = left + 'px';
}