Experimental aircraft pilot reported the aircraft had intense engine vibrations that led to vital components coming off the aircraft that then resulted in an off-airport landing. The aircraft was having problems days before the flight that seemed to be resolved; but the pilot stated the crankshaft failure may be unrelated.

Narrative

At around XA:40; I departed ZZZ1 with the destination of ZZZ. My plan was to spend the night in ZZZ and depart the following day back to my home airport ZZZ2 where the aircraft is hangared. I was the sole occupant on board; was not on flight following; and had not filed a VFR flight plan.The engine ran well; all temps pressures in tolerances and no abnormal vibrations. About 2 hours into the flight and 20 miles southwest of ZZZ at 3;500 ft. and XC:30; I noticed slightly more vibration than usual. I adjusted the power and mixture settings and tried running on either ignition. While doing this the vibrations gradually worsened. I reduced the power and switched frequency. The vibrations had increased to the point that the instrument panel was no longer readable. After approximately 2 minutes since the onset of vibrations; the crankshaft cracked at the front main seal and the propeller; spinner; and flange assembly broke off without hitting any other part of the aircraft. Without the propeller; the engine spooled up and the vibrations immediately ceased. I [requested priority handling] and glided to a road directly below me. I was about 17 miles southwest of ZZZ when the propeller departed.I landed safely on the road without hitting anything on the ground or doing any damage to the airframe. After I came to a complete stop some motorists helped me move the plane off the road to a gravel parking lot around 100 ft. from where I stopped. About 10 minutes after landing; local authorities arrived and took down my information. I called ZZZ4 Clearance Delivery and let them know I was down safe.History leading up to the immediate landing flight:The original plan for the trip was to fly from ZZZ2 to the ZZZ5 to visit some friends for the weekend. On Day 0; a friend and I left ZZZ2 where we made it to ZZZ6. After landing; while doing a mag check we noticed that the engine ran rough on one of the ignitions. We ended up stopping for the night to troubleshoot. Here I cleaned the spark plug leads and resealed one of the intake manifolds with RTV (Room Temperature Vulcanizing). The other intake manifold had leaked about 2 weeks before; which was fixed by sealing the connection to the cylinder head with RTV. The next day Day 1 we started up and had no issues running on either ignition and we continued on our way. On this day; we encountered no issues until we were passing by the ZZZ4 where about 1.5 hours into a flight we encountered an occasional stutter of the engine; as if fuel flow was being interrupted for a split second. The occasional stutters began to increase in frequency; about once every 30 seconds. We diverted to ZZZ7 to investigate. Here we spent 2 days troubleshooting. I checked and gapped all of the spark plugs; crimped the spark plug leads to get better contact with the plugs; and finally tuned and cleaned the carburetor. After a solo flight consisting of an extensive run-up and 2 laps in the traffic pattern; the issue was seemingly resolved.On Day 3 we decided to start heading directly back to ZZZ2. Once again we encountered the occasional stutter of the engine about 5 minutes into the flight. We landed at ZZZ8 to investigate. After landing we also once again encountered the issue of the plane running rough on the same one of the ignitions. Here we consulted with some aircraft mechanics at the airport who suspected that the faulty ignition may have a short somewhere; which could explain our momentary engine stutter in cruise as well as one of the ignitions running rough. We insulated and heat shielded the spark plug wires; and checked that each one was sparking at the correct timing. After this; both ignitions were once again running smoothly. After another extensive ground run and a solo lap in the pattern we continued on our way on Day 3.For the 3rd time we encountered the occasional split second power loss in cruise and made the decision to land at ZZZ1. The following morning; Day 4; I consulted with anotherAircraft Mechanic and reached out to the [internet] forum for assistance. The forum almost unanimously agreed that the split second power loss issue was due to vapor lock in the fuel lines as they are directed close to the hot exhaust pipes. I further insulated the fuel line and gascolator with insulating foil tape; and ensured they were not close to contacting the exhaust pipe. I also drained and replaced all the gas. After this; I made a 40-minute solo flight east of the airport where I underwent several climbs; descents; and straight and level flight at a range of power settings. The engine ran smooth and uninterrupted for the entirety of the flight. This ultimately led me to the conclusion that the momentary engine stutters were due to vapor lock; and the rough running ignition was due to a spark plug lead wire shorting. Both issues were to my knowledge now fully resolved before I departed on the flight. At this point my friend and I decided it would make the most sense for me to continue solo for the remaining flights back to ZZZ2.At around XA:40 I departed on the flight from ZZZ1 to ZZZ. Throughout the flight I encountered smooth engine operation on both ignitions; running at normal temperatures and pressures; and no vapor lock issues. This flight continued until I experienced the; to my knowledge; unrelated crankshaft failure and immediate landing.