Here is some useful information about Fines for motoring offences in Spain. Read more here
New fine rates have recently been published, and over set limits in each location (autopista, town, etc.) you can be arrested on the spot. You are not allowed to have a radar speed detector in your vehicle, let alone use one. Speed traps are becoming quite frequent but not as bad as in the UK with cameras (yet; the salesmen are moving in). Fines for other offences are calculated on the severity of the offence and there is a table for the guidance of the police and Courts.
If you are a tourist without assets in Spain, all fines are payable in cash “on the spot”. As well as that you get choice of doing a community works as well as pay your fine in 12 month. The legal drink-drive limit is currently 0,5 grammes per litre of air using a breathalyzer. The very high death rates in Spain (in the top 3 in the “old” EU) means that if caught with excess alcohol or drugs in your body, you can expect to lose your licence (in a special Court, possibly that same day) or, if a resident, have to attend a special school.
A common fine currently being issued is for not having lights on when it’s raining. The fine is around 800 € and 4-6 lost of points of Spanish driving license, if you have one.