Ulf Grönholm, two-time Finnish national rally champion and the father of 2000 & 2002 WRC champion Marcus,
traffic accident, 1981. He and his co-driver
Bob Rehnström died in a traffic accident on 26th of February, 1981 at Kirkkonummi while practicing for Hankiralli (modern-day Arctic Lapland Rally). Translated from Finnish Wikipedia article (with listed sources):
Grönholm and co-driver Bob Rehnström died while doing final preparations for Hankiralli in the middle of the night, 26th of February 1981, in Kirkkonummi. Grönholm collided with a snowplow that was stopped with lights closed on the road, driver sleeping in the cabin after a long day. The Fiat piloted by Grönholm burst into flames. Both Grönholm and Rehnström died upon impact. The road was not closed for practice.
In other words, it was an illegal (yet common at the time) practice run for an upcoming rally, driving at race speeds on public roads during night.
------------------------------------------------------------------------------------
Carl-Otto Bremer, his co-driver
Juhani Lampi and co-founder of Hankiralli
Lauri Hurme,
plane crash, 1962.
Translated from Finnish sources:
Carl-Otto Bremer first found success in slalom before transferring his skills to motorsport in 1956. On the following year, he was 2nd at Midnight Rally in Sweden and 6th in the Thousand Lakes Rally (later known as Rally Finland/Neste Rally Finland). In 1959, Bremer won the over 2-litre sports car class at Eläintarhanajot. His success would continue in 1960, as he won the Finnish national rally championship, Thousand Lakes Rally and placed 9th at the Monte Carlo Rally. He was selected as the Finnish racing driver of the year in 1960. In 1961, he won the Winter Rally at Norway.
Bremer piloted a Saab throughout his rallying career, and is credited for creating its rallying reputation which followed the Swedish marquee throughout 60s and 70s.
On 7th of August, 1962, the plane piloted by Bremer crashed into the shore of his summer cabin at Snappertuna with fatal consequences: Bremer, Juhani Lampi and Lauri Hurme all passed away.