Bike rain poncho plus fenders:

Rain Capes from People Who Really Know Rain Capes!

Vaude Covero Poncho II - Outerwear | Adventure Cycling Association

They not as urban chic as FBinNY's garbage bags but they're in much brighter colors than most garbage bags which makes them good for low light conditions. Rain ponchos need not be made out of uberexpensive "breathable" laminates since they're open underneath and you can sweat just fine with them.