Roads are of course not designed to be more dangerous that is absurd; which cash-strapped council wants to try their hardest to be sued? Here are a few things to bear in mind anyway:
Many road improvement schemes are designed to re-prioritise non motorised traffic and slow down motorised traffic to make it easier for less mobile people to get around; streets are no longer (and haven't been for a while) designed for the sole purpose of conveying motorised traffic. (well, some obviously are like motorways, but city 'streets' are now designed with pedestrians and cyclists as the most important road user. This is to make travelling by sustainable modes easier which is a key policy target from EU law/policy and down) These days, providing a highway improvement scheme will often compromise something for one type of road users to give a benefit to another type road users. if you're having a particularly slow Sunday afternoon, have a look at Manual for Streets, which sets out the principles for street design in 21st Century UK. It is a free book and its readily available online.
Also note that no road improvement scheme is perfect, because of various constraints such as land availability and budget constraints, therefore some risk will always remain, just like when you walk outside your front door every morning! I am not a safety engineer so I don't know where the line is drawn in terms of "this is not safe enough" or "this is not actually an improvement".
When a street improvement is designed it goes through 3+ stages of Road Safety Auditing. Stage 1 and 2 occur during the design process and 3 and 4 occur once it is built. These are in theory independently undertaken and specifically assess a highway design for safety, for all road users. The Safety Audit checks for issues and recommends solutions. An "approved" audit is usually a requirement for the scheme to go ahead.
Finally, normal people who use the roads every day are not highway engineers. Usually when people are clamouring for a roundabout to be installed they don't know why that kind of junction is unsuitable, or how many road users would be negatively effected by a change in junction type, or what the constraints are that stopped a different design going ahead etc... People don't seem to realise that usually a lot of thought goes into highway design! There is also the mix of political and engineering at work; local people (and therefore a local councillor) will be clamouring for a certain (but in engineering terms unsuitable) improvement, the highway authority responsible for providing or not providing said improvement (usually a County Council or Unitary Authority) is essentially a political body so...
However, there are of course poorly engineered designs highway designs which could have been avoided which stem from mistakes being made, or lack of forsight etc...
I work (and worked) alongside highway engineers both in the private (and public) sectors. I should also say that I don't necessarily agree with all of the current approach.