Signs to indicate the legal rate of speed and that the speed of motor vehicles may be measured by radar or other electrical devices shall be placed at or near the State boundary on those interstate and primary highways which connect the Commonwealth to other jurisdictions at such locations as the Commissioner of Highways, in his discretion, may select. There shall be a prima facie presumption that such signs were placed at the time of the commission of the offense of exceeding the legal rate of speed, and a certificate by the Commissioner of Highways as to the placing of such signs shall be admissible in evidence to support or rebut the presumption. Such legal rate of speed and notice of measurement of speed by radar or other electrical devices may be posted on different signs and need not be posted on the same sign.