Narrative:

ATC issued a 'descend and maintain' instruction. Upon reducing throttle to comply; the aircraft engine began running very roughly and then promptly quit.with a known 70+ gallons onboard and both main tanks full; very little time was spent troubleshooting and I immediately [advised ATC] and turned towards ZZZ. ATC provided ample assistance; including an alternate airfield that would likely be less impacted by adverse weather in the area. It was immediately clear that both ZZZ and the ATC-suggested field would be at the outer limits of my available glide distance. ATC and I worked to create another plan; which included flying in the direction of ZZZ while following a highway as a landing alternative.at approximately 4nm southeast of ZZZ; running out of straight highway and coming upon the populated edges of the town; I decided to make an off-airport landing on the highway. The landing required a course reversal to land to the east to make use of the straight sections of the highway.a semi/tractor-trailer was in opposing traffic and I had sufficient altitude to allow this vehicle to pass underneath before I dropped gear and flaps. However; a steep descent rate was required as a bend in the highway was coming up. This increase in airspeed put me at odds with same-direction traffic which I was gaining on rapidly. Holding off touchdown as long as possible allowed a reduction in airspeed and a safe landing without injuries to pilot or ground persons and no damage to property.ground diagnosis identified that sufficient oil was present (9.5 quarts); that the prop turned through (not seized) and that the electric fuel pump provided fuel from each main tank to the engine (fuel was present in both tanks). The final cause of the failure will require additional analysis.as to how I could have possibly improved on the outcome? If more time (even just an additional 30 seconds during the controlled descent) had been spent working the checklists; I may have been able to isolate the problem through a combination of alternate fuel tank; electric driven pump or magnetos. If successful; this may have been sufficient to extend the flight safely to ZZZ; averting a highway landing. As it was with the failure occurring on the shoreline and with just 9 minutes from failure to touchdown; I did not focus on in-air troubleshooting but instead focused my mental faculties on identifying the safest available place to terminate the flight and then on flying the airplane to that point. I believe in retrospect that time spent 'inside the plane' during the early minutes of the emergency would have resulted in missing the highway; possibly requiring landing in a wooded area; a swamp or at least in a water-logged field.my hope is that; by sharing this story; other general aviation pilots will place increased emphasis on committing emergency checklists to memory and regularly practice them in such a way that they are as automatic as possible when faced with an actual emergency. As the saying goes 'I'd rather be lucky than good'; but since luck also favors the prepared; I will be working on increasing her favor while also evangelizing the same to other GA pilots.

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Original NASA ASRS Text

Title: BE-35 pilot reported an engine failure while cruising at 9;000 feet; resulting in an successful landing on a highway.

Narrative: ATC issued a 'descend and maintain' instruction. Upon reducing throttle to comply; the aircraft engine began running very roughly and then promptly quit.With a known 70+ gallons onboard and both main tanks full; very little time was spent troubleshooting and I immediately [advised ATC] and turned towards ZZZ. ATC provided ample assistance; including an alternate airfield that would likely be less impacted by adverse weather in the area. It was immediately clear that both ZZZ and the ATC-suggested field would be at the outer limits of my available glide distance. ATC and I worked to create another plan; which included flying in the direction of ZZZ while following a highway as a landing alternative.At approximately 4nm SE of ZZZ; running out of straight highway and coming upon the populated edges of the town; I decided to make an off-airport landing on the highway. The landing required a course reversal to land to the east to make use of the straight sections of the highway.A semi/tractor-trailer was in opposing traffic and I had sufficient altitude to allow this vehicle to pass underneath before I dropped gear and flaps. However; a steep descent rate was required as a bend in the highway was coming up. This increase in airspeed put me at odds with same-direction traffic which I was gaining on rapidly. Holding off touchdown as long as possible allowed a reduction in airspeed and a safe landing without injuries to pilot or ground persons and no damage to property.Ground diagnosis identified that sufficient oil was present (9.5 quarts); that the prop turned through (not seized) and that the electric fuel pump provided fuel from each main tank to the engine (fuel was present in both tanks). The final cause of the failure will require additional analysis.As to how I could have possibly improved on the outcome? If more time (even just an additional 30 seconds during the controlled descent) had been spent working the checklists; I may have been able to isolate the problem through a combination of alternate fuel tank; electric driven pump or magnetos. If successful; this may have been sufficient to extend the flight safely to ZZZ; averting a highway landing. As it was with the failure occurring on the shoreline and with just 9 minutes from failure to touchdown; I did not focus on in-air troubleshooting but instead focused my mental faculties on identifying the safest available place to terminate the flight and then on flying the airplane to that point. I believe in retrospect that time spent 'inside the plane' during the early minutes of the emergency would have resulted in missing the highway; possibly requiring landing in a wooded area; a swamp or at least in a water-logged field.My hope is that; by sharing this story; other general aviation pilots will place increased emphasis on committing emergency checklists to memory and regularly practice them in such a way that they are as automatic as possible when faced with an actual emergency. As the saying goes 'I'd rather be lucky than good'; but since luck also favors the prepared; I will be working on increasing her favor while also evangelizing the same to other GA pilots.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.