Experts say attacks in which cars or trucks are driven into popular pedestrian areas present a unique challenge for law enforcement officials as they are nearly impossible to predict and easy to pull off. Four people were killed and dozens wounded Wednesday in London with this tactic — the worst attack on British soil since the transport network bombings on July 7, 2005. Charlie Winter, a senior research fellow at the London-based International Centre for the Study of Radicalisation and Political Violence, says what makes such attacks so frightening is the relatively low barriers to entry. Rita Katz, director of the SITE Intelligence group, says it is almost impossible for law enforcement agencies to stop IS-inspired attacks, especially vehicular-style ones like the one in London. Since 2014, this simple but effective attack has been promoted in IS propaganda online. Two weeks later, an Ohio State University student rammed his car into pedestrians on campus and then got out and started stabbing people with a butcher knife before being gunned down by a police officer. The devastating potential of such violence was dramatically illustrated last summer in the French beach town of Nice when a cargo truck took to the crowds celebrating Bastille Day in an attack that left 86 people dead and hundreds wounded.