Suggestions for prevention derived from the "situational" research that has been done can be divided into measures that reduce the physical opportunities for offending or increase the chances of an offender being caught. In the case of opportunistic crimes (ones apparently elicited by their ease of accomplishment), it is logical to assume that the probability of offending could be reduced by making it more difficult to achieve the crime or to increase the chance of being caught. In practice, increasing the chance of being caught usually means attempting to raise the chance of an offender being seen by someone who is likely to take action. There is evidence that "defensible space" designs on housing estates give some protection from vandalism. There are some objections to the concepts and practices of attempting to reduce crime by making it more difficult for criminals to achieve their crimes and increase the chance of their being caught. Some argue that it fails to address the root causes of crime and will only displace crime from more difficult targets to easier targets without reducing the overall crime rate. Although these objections may have merit in some cases, there is evidence that certain types of crimes have been reduced through comprehensive efforts to reduce opportunities for their commission, such as baggage-screening measures to reduce airline hijackings. Criminals do make rational choices about what targets to attack, and there is evidence that the more difficult crimes are to commit without being caught, the fewer the number of crimes committed, even though there may be some displacement to easier targets. 50 references