No one denies our roads are in a mess, in many areas congestion to the point of regular jams. The planning policies of the late twentieth century lead to a complete focus on privately owned motor vehicles as the primary mode of transport. This has been exacerbated by the high cost, complex fare structures and relative discomfort and inefficiency of public transport.
The problem is now that the proportion of the growing population that own motor vehicles is increasing. Our roads are pretty much at capacity in many places.
Road safety (especially for drivers) is improving. Probably this is due to improvements in car technology, safety belts/air bags, and the decreasing opportunity to travel at high speeds due to congestion. Its fair to note that safety for vulnerable road users, pedestrians (especially children), cyclists, and especially motorcyclists is not improving as much as for drivers.
Cars are becoming larger and more powerful. I need to check, but I suspect that the width of C21st cars is greater than the width of many cars 20 years ago. I suppose this partly the influence of the US, which still uses much larger cars than any other country, and the globalisation of the car industry (and many countries do not have the compact streets of European cities).
UK streets are almost entirely given over to cars. Cycle lanes are actually part of the problem as they are mainly used as expansion areas for cars. On my cycle journeys I've never been able to traverse several cycle lanes without finding them occupied by parked cars, or intruded into by drivers (both quite legal). Bus lanes are one of the few instances of areas where private cars are not allowed, however these are not universal and often not popular with drivers.
UK drivers often use cars (designed to carry 4 or 5 people) to transport one person. About 60-70% of car journeys have one person on board. It wouldn't be a problem if cars were smaller, but each person now takes up the space of 4 or 5. (But if you bought a car why not buy one big enough for as many people as you are ever likely to carry?).
UK streets are often clogged up with on street parked cars, often leaving just enough space for two cars, or often only one car in the lane. Where parking is not allowed (double yellow lines) drivers still dally, often misquoting the rules pertaining to the 20 minute 'loading' allowed even on double yellow lines.
Cars are very powerful compared to the models a few years ago (they are also much more efficient in terms of fuel use). My second car, a Triumph Herald, 1965 model, had an 1147cc engine, generating 39 bhp, doing about 36 mpg. My current car is at the economical end, its a Toyota Yaris Verso, 2000 model, has a 1497cc engine, generating 86 bhp, doing about 46 mpg.
Cars are much more isolating than they were. Air-conditioning, sound proofing, monocoque construction (more rigidity and less vibration), and safety equipment isolate drivers from the risks of driving.
Poor driving. Perhaps not a greater proportion of drivers, but the levels of congestion mean that they are more frequently encountered. They cause massive amounts of chaos.
The feed frenzy focussed on drivers. From parking to the stopping of innocent drivers to collect fines. Clamping and 'over parking' charges that actually add to the dangers on the road (for example restrictions on motorway parking to increase turnover).
Cities are often traversed by uncrossable roads, with pedestrians having to take long ways round, often climbing bridges, or taking unpleasant underpasses to get by.
People choose to live so far from where they work, and become reliant on using a car. 100 years go many people never travelled more than 30 miles from home, in the C21st many people travel more than that every day to work...
The roads we use will become unusuable. Cars are very beneficial in some situations (disabled, elderly, families with young children, for carrying large loads), but those benefits will be lost by the gross over use of cars for short journeys and single occupancy transport. Roads become more congested and difficult for alternative modes of transport. Its currently faster to travel by cycle than car in the city I live in, but most people won't try that because of the subjective danger. Public transport on the roads fails because of the levels of traffic. I think there is no choice but to take more road space from drivers for public transport, and for cyclists, but its not going to be easy. We need to address car design (and car ownership), so it better reflects the types of journeys people want to take (there need to be some incentives for buying smaller cars for example - disincentives from buying larger ones). We need to increase subjective safety of walking and cycling. We need to address planning of roads, housing, and work in order to make non-car transport feasible and more attractive.
Of courses its all impossible isn't it? Except that is how some countries do it, Holland and Amsterdam being two obvious examples.
...and currently many countries in the world are racing into a UK like problem. Cycling and walking is what poor people do. The people making planning decisions are the ones driving the large black imported US cars. Don't do it!