It is known that the current congestion calculation, whilst still an improvement, is a huge compromise as the relation in between congestion and travel times is quite guessed.

Currently, congestion is defined as the proportion of #vehicles/#stopping_vehicles on a tile.

A congestion of 100% expects the journey time to be doubled.

I had some observations of how well that guess matches the actual travel times and came to the conclusion that it's slightly overestimating the affect for not-really congested roads used by mixed speed vehicles e.g. 75 km/h busses and 88 km/h busses, but in any real congestion situation underestimates the affect on journey times by-far!

I had observed a 6km long congested stretch of road in betweeen in between swainsgrave and chorborough.

The congestion of each tile was around 30%, roads and vehicles maximum speed being 96 km/h

Thus, the congestion calculation would expect an uncongested journey time of 3.75 minutes, added 30% journey time due to congestion, we end up at ~5 minutes.

So let's get to the actual journey time of multiple observed vehicles:

The measured journey time for that stretch of road was for 9 out of 10 observed vehicles in between 35 and 40 minutes, thus roughly 10 times the guessed travel time!

However, that road is quite a hard one, so I also had further such observations around the intersection where the Burnerminster bypass joins the Burnerminster road.

I have observed 4km of road for both of these.

Burnerminster bypass is a 64 km/h TarMc, congested by ~40%, Burnerminster road is partly TarMc congested by ~60% and hot rolled asphalt congested by ~50%

Calculated journey times:

Burnerminster bypass: 3.75 minutes uncongested, 5.25 minutes congested

Burnerminster road: 1.4+1.5625 minutes uncongested, 2.24+2.34 =~4.6 minutes congested

Observed:

Burnerminster bypass: 9 minutes

Burnerminster road: 13 minutes

So, what does this mean?

As expected, the current congestion system does not at all relate to the actual travel times.

A fairly small congestion value can cause very huge delays in travel time, as can be seen at swainsgrave-chorborough.

Further, road ane vehicle max speeds do not well relate to congestion speeds.

Our current expactation that a congestion of 100% would double the journey time by-far underestimates the effect.

Suggestion:

In the long term we should think about the average tile speed based congestion calculation.

In the short term, we should increase the effect of congestion to the guessed jorney. "A congestion of 100% will multiply tiles expected jorney time by 5". Even that will underestimate the effect in most cases, but I think it's a good interims solution.