rate, and relatively low proportion of accidents due to fatigue or inattentive driving would have questionable value. Diminishing marginal returns In addition, safety improvements are not linear; there are diminishing marginal returns with a safer driver population, in which it is more difficult to further reduce the accident rate. Within the industrialized countries the rate varies between about 8 and 27 (per 100,000 licensed drivers per year). "Safety improvements are usually subject to the law of diminishing marginal returns. This means that for every improvement of a fixed amount, the safety benefit gained decreases a little each time. For example, increasing the width of the median from 50m to 60m will decrease the number of collisions less than increasing it from 10m to 20m. Eventually, a width will be reached at which widening the median further cannot be justified because the improvement in safety is too small." When the accident rate is close to the baseline of 8, there may already be several factors pushing it down so adding another safety factor (initiative) will only yield a very small improvement. Installing rumble strips on a highway with a high accident rate close to 27 should yield a relatively high accident reduction. This assumes that the road shoulder is adequate for a recovery, once a straying driver has been alerted by the rumble strips. Types of rumble strips Continuous shoulder rumble strips (CSRS) Montana undertook an extensive 10-year multi-site study of the effectiveness of CSRS on Interstate and primary highways (both types are divided pavements). This study also investigated the severity of crashes, which sets it apart from previous studies. The results indicated a 14% reduction in crashes on Interstate highways; however the effectiveness on primary highways indicated both improvements and worsening, and the results were considered inconclusive. It was found that "roll-overs" decreased in number, but increased in severity. The study only considered crashes in dry and wet conditions, not snow and ice. The FHWA undertook a multi-state study involving test sites from Illinois and California. The Illinois component indicated crash reduction from 7.3% to 21.7%. The California component indicated crash reductions of 7.3%. This study also indicated an overall reduction of about 14%. The 1997 New York State Thruway study indicated a 65% to 70% reduction. However, in a 1999 New York Times article regarding the New York State Thruway study, an official stated that the experiment was not