What would cause my car to lose electrical power when driving on freeway?

When driving my 1992 Honda Accord on freeway it seemed like all my electrical seemed to be going... headlights were dim, radio wouldn't work. As soon as I used my turn signal, my car almost died. What would cause this? I have been told battery, alternator and fuel pump... but fuel pump seems unlikely because it seems purely electrical? Any help would be appreciated. Thanks!