Before the twentieth century, most Americans rarely came into contact with police officers. But with more and more drivers behind the wheel, police departments rapidly expanded their forces and increased officers’ authority to stop citizens who violated traffic laws. The Fourth Amendment—the constitutional protection against unreasonable searches and seizures—did not effectively shield individuals from government intrusion while driving. Instead, jurists interpreted the amendment narrowly. In a society dependent on cars, everyone (the law-breaking and law-abiding alike) would be subject to discretionary policing. Sarah Seo's remarkable book Policing the Open Road shows how procedures designed to safeguard us on the road actually undermined the nation’s commitment to equal protection before the law.