Today, if you buy a car its emissions are many orders of magnitude smaller, than the one from 50s. The most likely it will emit much less than the current allowed levels, which go down every year. If you look into the near future still there is a lot which can be done and is going to be done. Also there are works in the area of city vehicles, which are optimized for city cycle, that is, for example, frequent accelerations or long waiting in traffic jams.
Buses improved a lot too. They look prettier, they are much less noisy, they use less fuel and are more pleasant for their passengers. A disabled person can usually access it very easily.
Finally fuels have improved a lot. Leaded petrol is no longer used and diesel fuel has much less contamination, such as sulphur. Also all new cars and buses use multi stage catalysers which significantly reduces the level of unwanted emissions.

But ... still the average British car is 7.7 year old. A bus probably more than that. Many can hardly stay within the emission limits ...

How about bikes. A bit this, a bit that. Perhaps look a bit fancier but nothing really changed. 50 years ago cars needed attention every hundred km or less and bikes roughly the same. Today, an average car is likely to work fine for 60 000km or so without any trouble, just with recommended servicing. Today's bike needs attention every 500-1000km, that is only about 5 times more than its older brother from 50's. There are some kinds of bike "mutants" such as recumbents. They are much more expensive (usually 5+ times) but can give much greater protection to the cyclist in case of an accident. However ... not popular at all. The high cost and much lower "portability" (more difficult to transport by car or bus) can be blamed for their low popularity.

As you can see bicycles are being developed at much lower pace than cars. Obviously, there will be one day when cars "overtake" bikes in all the fields.

Why ? It is impossible! Bikes are green! No, as long as they do not grow on trees. At the moment during the production of one bike about 130kg of CO₂ from non-renewable sources is being released. Not a lot but that bike is also used not a lot or at least much less than a bus or a car. Obviously many other gases are emitted too.

But when this may happen? In my opinion we will not have to wait too long. Already in a few areas bicycles are well behind cars and buses.
 

OK, now please click on the following links to see the full details:

1. Conventional bicycles are simple, cheap, dangerous and inefficiently used man powered vehicles - cyclists tend to compare bicycles with cars or buses on one to one basis, what gives a very false image. Also fabrication emissions are usually ignored.

2. Road cycling is very dangerous, significantly more than using public transport or driving a car. On pavements cyclists are more dangerous to the others than the other to cyclists.

3. Bicycles offer only 2-3 times lower the overall emissions than cars or buses. Cyclists usually ignore the fact that somewhere in Asia there are acid rains caused by plants (steel, power,...), which have produced their bikes. However, after adjusting the emission values due to road incompatibility of bicycles, overall emissions of cars are lower than bicycles.

4. Cycle lanes are over 5 times less efficient, in terms of passenger flow, than bus lanes.

5. Cyclists are the road users the most exposed to exhaust gases, therefore cycling is potentially dangerous for the cyclists' health. In contrast to that, new buses have most of the time fume free air inside. Vibrations generated whilst riding a bicycle are proved to be dangerous for the cyclist's health.

6. Converting parts of heavily used city roads into cycle lanes is simply promoting a very unhealthy and dangerous way of travelling.

7. Cycling is not for everybody - only a small portion of population potentially can cycle on a road safely.

and some eco-nonsenses:

8. In the UK, during the rush hours, taxis, polluting 2 times more than average cars and much more than buses, are privileged to use the bus lanes.

9. In the UK the government is going to impose a carbon tax on the new cars only, that is the cars which are MUCH, MUCH more environmentally friendly that the one already on the roads. I suppose this is to encourage people to use more the old and smoky ones.