First off, you'd need an accurate field test for pot, similar to a breathalyzer, instead of having to wait for a lab analysis of urine/blood/ hair.  There are the quick tests that will indicate it in the system, but they have to be lab confirmed to carry any legal weight.  I've drug tested for over 20 years in the construction trade, and they get better every year, but none are anywhere 100% accurate.
Legalize it, tax the hell out of it (like alcohol) and sell it in liquor stores same as booze.  Restrict it's use and posession to adults only, (18 or 21) and prohibit posession/usege by minors.  Quit criminalizing simple posession, and  controlling it's ues as I've laid out above, and you could :
1) reduce crime by 
a)making it legal to posess/be under the influence of in certain circumstances
b) eliminating the criminal element in selling it
c)reduce the theft rate because price would drop
d)reduce the smuggling of it into the country
2)Reduce the number of people in jail/prison for minor pot related offenses, saving a LOT of money spent to house these "criminals".
3)Raise a LOT of money for governments thru the taxation of the drug.  Federal taxes are already in place on pot.  State, county and city taxes could be levied as well.
4)A good crop for farmers to make a ton of $$ growing.  Nice cash crop with a high demand.
You'd need serious penalties (just a with alcohol) for driving under the influence, but you'd need to be able to test the difference between what someone smoked last weekend, and what the smoked 1/2 hr ago.  Use on the job would also be regulated, just as with alcohol, since pot smokers tend to be a bit less careful.
As already mentioned, lots of people are killed because of alcohol related incidents.  Many people become mean drunks, and look for fights, often resulting in death.  I've never seen a pot smoker get mean, just develop a bad case of the munchies...HEY, tax munchies too!!
I haven't smoked the stuff in almost 20 yrs, but I'd still be in favor of legalizing it with some controls like I laid out above...