When I used to jog or bike along the road I'd carry a bottle of 10% ammonia to spray at aggressive dogs. Don't aim at their eyes, otherwise go for it. Less cost and hassle than the pepper spray; and in some areas its not considered legal/ethical to carry pepper spray unless you meet certain criteria:
Pepper spray is LEGAL in all fifty states. However, some states and cities have specific regulations pertaining to the purchase, possession and use of defense sprays. Please check with your local law enforcement agency for any regulations your state may have.
New York: the sellers must be local licensed firearm dealers or local pharmacies.
Massachusetts: the buyer is required to obtain a Firearms Identification Card (FID); the sellers must be local licensed firearm dealers.
Michigan: OC Pepper Spray can be no stronger than a 2% concentrate. CS is the only Tear Gas accepted. Both must be no larger than 35 grams per can. No combination sprays allowed.
Wisconsin: Pepper spray only. No Tear Gas, UV Dye or combination sprays. OC Pepper Spray can be no stronger than a 10% concentrate. The can must be between 15-60 grams only. Must have safety feature.
California: Canisters cannot be more than 2.5 ounces.