Frequent sender of such E-mails chiming in here:

Often when there is a response expectation, I'll explicitly say so in the subject, so that if you're scanning your E-mails it will stick out. Will also specify my default assumption should you not respond. I like to be super clear. Company culture is the product of the people who participate in company culture.

SUBJ: [Reply needed by 5PM] Deadline for project approval approaching

(somewhere in body): Blah, blah, blah. If I do not hear back, I will assume we will go forward with the project.