In my experience, at least most of the state traffic laws in the US hold that if a vehicle is hit from behind by another, the driver that did the hitting is assumed at-fault, unless extenuating circumstances exist (first driver stopped/reversed in a way that prevented the second driver from being able to reasonably react safely).
This is why guidelines regarding safe following distance exist. The second driver usually has no idea why the first would have stopped- could there be a child in the road? Something dangerous, like a fallen electric pole or spilled gasoline? Is the first driver supposed to just plow through that too?
Here we go, straight from Massachusetts regulations [http://www.mass.gov/?pageID=ocaterminal&L=5&L0=Home&L1=Consumer&L2=Insurance&L3=Automobile+Insurance&L4=Safe+Driver+Insurance+Plan+(SDIP)&sid=Eoca&b=terminalcontent&f=doi_BOA_Boa_sof_on&csid=Eoca]:
"(03) Rear End Collision. The operator of a vehicle subject to the Safe Driver Insurance Plan shall be presumed to be more than 50% at fault when operating a vehicle which is in collision with the rear section of another vehicle."
Stopping in order to not hit an obstacle on the road would be considered safe driving. Stopping in order not to hit a live animal would be considered humane.
This is why guidelines regarding safe following distance exist. The second driver usually has no idea why the first would have stopped- could there be a child in the road? Something dangerous, like a fallen electric pole or spilled gasoline? Is the first driver supposed to just plow through that too?
Here we go, straight from Massachusetts regulations [http://www.mass.gov/?pageID=ocaterminal&L=5&L0=Home&L1=Consumer&L2=Insurance&L3=Automobile+Insurance&L4=Safe+Driver+Insurance+Plan+(SDIP)&sid=Eoca&b=terminalcontent&f=doi_BOA_Boa_sof_on&csid=Eoca]:
"(03) Rear End Collision. The operator of a vehicle subject to the Safe Driver Insurance Plan shall be presumed to be more than 50% at fault when operating a vehicle which is in collision with the rear section of another vehicle."
Stopping in order to not hit an obstacle on the road would be considered safe driving. Stopping in order not to hit a live animal would be considered humane.