Answer:
Hortatory.
Step-by-step explanation:
When driving down the highway and one sees or reads a sign that states, "Save the planet—don’t litter." The technique of policy enactment being used is hortatory.
A hortatory technique is focused on encouraging, exhorting and urging people to do that which is right, acceptable and worthy of emulation by others.
In this scenario, commuters and road users are being urged not to litter the highway with thrash, waste, or dirt, in order to protect the planet from environmental degradation and pollution. This would help in conserving and mitigating risks posing as a threat to our dear planet and habitat.
Hence, a hortatory technique in policy implementation and enactment is primarily aimed at appealing to the sense and conscience of individuals, to engage in socially responsible acts.