30 down to 20 for non-arterial single carriage way roads in town makes sense (ie residential areas) however if you were to impose a 20 limit on roads that actually take you anywhere you would increase speeding, increase frustration and probably at the same time increase accidents (drivers paying less attention cos they are going slower and are irritated, possibly even rat running to take the shortest route) .
A 40 limit on long distance journeys would be hell, as would trying to get on a train or plane as everyone would shift (remember buses can only go as fast as cars) to the faster forms of transport, the capacity of the rail network is limited so flying may become a bigger option for more people. Thus not only have you put up the accident rate (drivers paying less attention cos they are going slower and are irritated, possibly even rat running to take the shortest route) but the amount of CO2 being emitted has gone skyward as well (the planes).
It would also increase the amoutn of time the daily commute takes for those who take the bus or drive because the average speed would be lowered where car on car accidents are more likely (unless it is usually nose to tail in which case the lower limit may allow freeer flowing traffic in which accidents are more likely to happen with pedestrians)
Lets look at the change in commute times for bus pbuttengers
Journey 1:
I currently do a 12 mile journey by bus, the bus travels 2 miels where it can go at 50mph and 10 miels where it can do 30mph. It has to stop ocasionally of course. It takes roughly 40 mins
V=D-T = 12-0.66 = 18.2 Mph average
Since the 50mph section will be at the same speed T=D-V = 2-50 = 0.04 (of an hour) (2.4 mins) Change that to 5 to handle acceleration and braking time
So 35mins worth of an 18.2mph average is spent in 30 zones just to do 10 miles Sicne the bus is achieving 60% of the speed limit ofr its average speed, lets raise that to 70% for a 20 zone since we don't have to do as much acceleration to get to the speed limit or brake as early to stop at a bus stop. Thats 14mph Average
10 miles at 14mph. T = D-V = 10-14 = 0.7 = 43 mins
Thats buttuming the 50mph zone stays at 50 and doesn't become a 31mph zone
Route 2 (Actually Route 1 with Village and Bridge sections removed)
Journey from City centre to hospital takes 20 mins (0.33 of an hour) and is 5 miles on a No 75 bus. Average is 15.2 Mph which is 50% of the speed limit, so again adjust for reduction in accelerating up to speed and braking for stops 60% is 12mph
So that would make the journey take 25mins
So for a journey taking in a Village, a Road Bridge and a City you add 3 minutes in 40 (7.5%) And for a journey through the City you add 5 mins in 20 which is 25%
This is for a city which has the lowest car ownership in Scotland, and relativley free flowing traffic, the only snarl up being roadworks and traffic lights.
Niall
