The freeway system uses ramp meters to help keep traffic moving. WSDOT uses variable message signs to let travelers know if there is an accident, to tell drivers how long their drive will be to certain areas, and for Amber Alerts. Recently, WSDOT installed variable speed signs along SR 520 and I-90 between I-5 and I-405, and along I-5 between the West Seattle Freeway and SR 520. There are also HOV lanes to move buses and carpools faster on many freeways and arterials. The HOV lanes on I-405 allow general purpose traffic to use them after 7pm (till about 5am), and the lanes on SR 167 are actually HOT lanes (carpools may use the lanes free; solo drivers can use the lanes for a variable fee). There are 225 lane miles of HOV lanes built and another 100 unbuilt. Freeway improvements are paid for by two gas taxes, 2003 Gas Tax and 2005 Gas Tax.