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In the US, the transition altitude is standardized at 18,000ft, however in Europe it is different for every airport.
Why is it not standardized like it is in the US? I could see situations where someone has their altimeter calibrated differently, and thus giving different altimeter readings, which would seem dangerous, especially for smaller GA aircraft that might not have TCAS?
You're right that having a common transition altitude is better- efforts are underway in Europe to set a common transition altitude. Multiple studies have been carried out by Eurocontrol and it has been noted that it is better to have a single transition altitude. For example, from an aircrew prespective:
The multiplicity of transition altitudes and the national rules and procedures make the European environment very complex. This can lead to a lack of altitude awareness and altimeter mis-settings and is operationally unsatisfactory.
and
Establishment of a common transition altitude has a clear safety benefit.
Similarly, from the ATC perspective:
The establishment of a common TA for ECAC States and the EUR Region is a fundamental element in achieving the goal of a unified sky and the safety policy of reducing risks to the greatest degree practicable.
The reason such an unsatisfactory state of affairs have persisted is that people have become used to that- the second document makes that point:
ATC providers, as is human nature, grow comfortable with what they are most familiar.
The main reason is that there is no requirement for any common transition altitude. The relevant ICAO document simply states:
2.1.2.1 A transition altitude shall normally be specified for each aerodrome by the State in which the aerodrome is located
and
2.1.2.3 As far as possible, a common transition altitude should be established: ...
As a common transition altitude is not exactly a requirement (in-spite of its advantages) different European states have historically established their own TAs, to be decided by the aerodromes or the regulatory authorities.
As they have become comfortable using it (and no major accidents have happened because of this), there is no serious move towards the establishment of a common transition altitude (through arguably, this is the case with much of the world- common transition altitude is not that common). In the UK, CAA has launched a consultation process to raise the transition altitude to 18,000 ft, which notes that,
... it has been agreed that the TA in the UK will be raised to 18,000ft.
It is the pilot’s responsibility to know what the TA is where he is flying. Typically it is a fixed value for an entire FIR and/or country, which makes this relatively easy.
There is a buffer of at least 1000ft between TA and TL specifically to prevent the safety issue you describe (including allowable altimeter errors). Also, ATC’s terminology will tell you whether to set QNH or QNE, even if you didn’t bother to look up the TA as you should have. So, varying TAs simply isn’t a significant problem in practice.
While it would seem marginally easier for everyone to have a common TA, that disregards the reason why different TAs exist in the first place: terrain. A low, flat country can reap the benefits of a low TA, for both pilots and ATC, while a mountainous one can’t.
Also, not all of the “US” has a TA of 18kft. The US-managed Oceanic Control Areas, which aren’t that far offshore and include part of the Caribbean, have a TA as low as 6kft; there’s no reason for it to be higher over the oceans where there is no terrain to worry about—and often no available altimeter setting to safely fly below the TA, so the low TA allows a low TL and thus more controlled airspace for IFR flights.
