highways were due to the primaries having smaller shoulders than Interstates. Secondary highways are single-lane undivided highways, and CSRS would be expected to be less effective than on primary highways. The most serious problem would be an increase in crash severity. Also, there is the concern of drivers sometimes overreacting and crossing the centerline, resulting in a head-on collision. The recovery zone width and condition of single-lane highways can vary greatly. It appears that there may be no published before-and-after CSRS studies for single-lane highways. Actual vs. isolated CSRS and centerline rumble strips effectiveness Given behavior adaptation and migration, the current rigorous Interstate effectiveness of 14%, and CLRS on single-lane highways effectiveness of 14% could be over-estimations of the actual "big-picture" reduction. In certain situations, such as an engaging single-lane highways that typically have narrow shoulders, high precipitation, in a northern climate with frequent freeze-thaw cycles, rumble strip effectiveness may be negative. As before-and-after studies become broader and more sophisticated, it appears the reduction estimates for both CSRS and CRS are less impressive. This may be due to the initial installations were on highways that had been identified as having very high accident rates due to inattention. Also, there may have been other accident reduction campaigns in concert with rumble strip programs." Also, as lane departure warning systems built into vehicles become more widespread, the physical CSRS, CRS, and CLRS in pavements may become increasingly redundant. Shoulder width Research has found that on rural freeways, rumble strips are much more effective when placed at or near the edgeline than when placed closer to the shoulder edge. Edgeline rumble strips can be expected to reduce crashes by 28.8%, and non-edgeline rumble strips only reduce crashes by 8.9%. On two-lane roads, there is little difference in effectiveness between edgeline and non-edgeline rumble strips, with crash reduction factors of 39.2% and 41.9%, respectively. FHWA now recommends rumble strips on two lane roads if the edge of shoulder is more than from the centerline, especially if the road has high volumes, poor geometry, or a history of run-off-road crashes. The 2003 Montana study stated that in certain cases, the rumble strips may act only as a warning of an impending crash, and that sort of a situation is much more likely where less shoulder is available for recovery. Recovery zone condition A concern about highways with narrow paved shoulders is the condition of the