It's impossible to do realworld testing, you need consistent conditions otherwise the results aren't comparable.
Don't forget though that RDE (Real Driving Emissions) also comes in to effect alongside WLTP.
Under RDE, a car will be driven on public roads over a wide range of conditions using portable measuring equipment (PEMS). RDE will complement WLTP to ensure that pollutant emission levels, measured during the laboratory test, are confirmed on the road, making Europe the only region in the world to implement such mandatory testing.
Hence a lot of the modifications manufacturers are having to make, as real world emissions have to be the same a lab (within allowable limits) measured levels.
RDE step 1 (with a NOx conformity factor of 2.1) applied as of 1 September 2017 for new car types (models that are introduced on the European market for the first time). It will apply in September 2019 for all types.
RDE step 2 (with a NOxconformity factor of 1.0 plus an error margin of 0.5) will apply in January 2020 for new types, and then from January 2021 for all types.
A conformity factor is defined as a “not to exceed limit” that takes into account a margin for error, which is present simply because PEMS equipment does not deliver exactly the same results for each test. For example, PEMS are not as accurate as a full laboratory system so they will not measure to the same level of repeatable accuracy as a lab test.
In practice, car manufacturers must set their design objectives well below the legal limit to be certain of complying, so it will mean massive changes in the way manufacturers go about designing power trains and vehicles.