At least one person died in the UK as they were unable to get to the hospital.
At this point their crusade becomes unworthy.
I agree that reliance on oil isn't great but there isn't a realistic alternative quite yet and the world's infostructure is based around fossil fuels. It is changing and will change but loss of life and all the pain that comes with isn't the way to push an agenda.
I think the story you're referring to is of a woman who pulled over to help another woman whose car had broken down on the motorway and another car ploughed into them. This took place on a parallel road to the one Just Stop Oil were slow marching which caused congestion further back and meant that an ambulance took 40 minutes to get to those injured.
There's a few tangents I could go on, including the general underfunding of the NHS which has meant that there have been ambulance delays of hours, such as the woman who waited injured on the pavement for 6 hours in Wales. This is due to shortages of vehicles on the road, thanks to over stretched hospitals being full and unable to bring patients in from waiting ambulances. If the main issue was public health and safety, you'd think that the government might want to intervene there but instead of putting their efforts into making sure that ambulances and hospitals are funded well enough to meet the needs of patients, they've instead sought to change the law of the country in order to criminalise protesting, with at least one MP saying that Just Stop Oil should be considered a terrorist group (that's probably not been said so much in the past week or so). But let's ignore that and stick to the topic at hand.
Just Stop Oil's protests on the roads have a policy of allowing emergency services to pass, in theory the emergency vehicles should be able to filter through traffic if people make room for them, obviously conditions vary and it sounds like in this instance that didn't happen. But, it's worth saying that this kind of awful news would be being reported far and wide if it was possible to pin more terrible outcomes like this on Just Stop Oil - newspapers love writing about and reporting on bad news when it comes to the "Loony Eco Mob" and the government love to talk about how they're putting lives at risk. Because we don't all know of those events happening, we can probably conclude the evidence just isn't really there, since both the press and the government would be keen to weaponise it.
If we are to say that the actions leading to death should be enough to make us change what we consider acceptable behaviour, perhaps we ought to think about some other things too. The World Health Authority is saying that between 2030 and 2050, climate change is expected to cause approximately 250,000 additional deaths per year, from undernutrition, malaria, diarrhoea and heat stress alone.*
That's without getting into the flooding, the wildfires, etc. etc.
So, while you can point at traffic jams and say that because it's possible that it could contribute to the death of a waiting patient, and also explain that people slow walking can cause a traffic jam, it shouldn't be too hard to follow that fossil fuels lead to climate change, which the World Health Authority are saying will cause hundreds of thousands of deaths over the next 30 years.
The saddest thing is that none of those deaths need to take place. We could move away from fossil fuels but the truth is, we're not doing that at anything like the scale we need to be doing it at to not result in those deaths.
*
https://www.who.int/news-room/fact-sheets/detail/climate-change-and-health