Zero Emission Zones (ZEZs) such as Oxford's should be reworked with general access restrictions that address all the harms done by motor vehicles, not just tailpipe emissions. In particular, Oxford's fails to address either particulate air pollution or road danger.
The goals of Oxford's ZEZ are to "improve the air quality in Oxford and reduce traffic" by "encouraging people to switch to low and zero emission vehicles" and "making other positive changes to their travel behaviour".
But the ZEZ charging scheme only considers tailpipe emissions. It fails to address particulate air pollution, which is generated by electric vehicles at similar levels to petrol or diesel ones.
Although lightweight EVs emit an estimated 11-13% less PM2.5 than ICEV equivalents, heavier weight EVs emit an estimated 3-8% more PM2.5 than ICEVs. In the absence of targeted policies to reduce non-exhaust emissions, consumer preferences for greater autonomy and larger vehicle size could therefore drive an increase in PM2.5 emissions in future years with the uptake of heavier EVs. ("Non-exhaust Particulate Emissions from Road Transport", OECD 2020)
(It's worth noting here that, while the current legal limit for PM2.5 in the UK is 25μg/m³, PM2.5 levels under 10μg/m³ have significant adverse effects and the World Health Organisation recommendation is that they be kept under 5μg/m³.)
Microplastics from tyres are also a persistent environmental pollutant.
The ZEZ charging scheme also fails to address road danger, which is actually worse with electric vehicles because they are larger and heavier for the same capacity, and accelerate faster. And it fails to address other harms to the public realm, notably the space taken up by vehicles (moving and parking) and noise pollution.
There is a clear public benefit in encouraging people to switch to lighter and smaller vehicles, not just to lower emission ones. A large electric SUV may create as much particulate pollution as a small petrol car and certainly does more to make the city centre dangerous for people walking or cycling; this should be reflected in the charge it pays to enter the city centre. HGVs pose far greater risks, and should be excluded from the city centre with exemptions only for those that can demonstrate a need to access locations within it.
The ideal is a Pigouvian tax which captures all the negative externalities created by motor vehicles. This would involve charging based on size and weight as well as on emissions: weight would serve as a proxy for particulate emissions and size as a proxy for road danger. (These are sufficiently correlated that just using weights would probably make sense.) And petrol mopeds and motorcycles could be charged more because of their over-size contribution to noise pollution.
You could also use NCAP Vulnerable Road user (VRU) score as a (weighted?) pricing factor. Some relatively recent cars have terrible VRU scores.
https://www.euroncap.com/en/vehicle-safety/the-ratings-explained/vulnerable-road-user-vru-protection/
The NCAP scores only appear to take into account injury severity in the event of a collision, though. Vehicles also differ in how likely they are to cause a collision, either by direct involvement or by blocking visibility - here larger vehicles can be dangerous even when stationary or parked.
Presumably, you are in favour of electric cars because they can dramatically reduce noise pollution.
Also, your question about inherent road safety ignores features typical in EVs.
- They tend to have much better braking systems, not only because their brakes are geared to the weight of the car but also because they tend to have two other features
- regenerative braking which often kicks in before normal brakes.
- cruise control and pedestrian awareness features, both of which can reduce accidents to pedestrians and other vehicles. This kind of feature is good for both acceleration and deceleration.
EVs help with noise pollution, but only partly. In the inner city and residential areas, mopeds and motorcycles and a few cars deliberately modified to make noise are responsible for 90% of the noise harm. And on higher-speed arterial roads (>30mph) it's air friction and road noise that are responsible for most of the noise, not engines. (So even if all the vehicles on the Oxford ring-road were electric - which isn't going to happen for ten or twenty years - that would only marginally help with the noise fromm that.)