Yes, traffic flow (the number of vehicles traveling past a point during a
given time period) is maximized at 30-50 miles-per-hour (50-80
kilometers-per-hour). A somewhat higher value sometimes occurs on
grade-separated highways with minimum lane changing, and a lower value on
surface streets with traffic control signals. Above this maximum point the
"shy distance" required between vehicles in the traffic stream increases
significantly. For more information see discussion in the "Traffic
Congestion" chapter of "Transportation Cost and Benefit Analysis"
, or the "speed flow" curves in
traffic engineering manuals.
This means that there is a traffic engineering justification for traffic
speed management as a congestion reduction strategy, particularly on urban
By the way, another good source of information on the safety impacts of
speed management is the Australian study, C.N. Kloeden, A.J. McLean, V.M.
Moore and G. Ponte, "Travelling Speed and the Risk of Crash Involvement,"
Federal Office of Road Safety, CR 172
, 1997, and thir more recent
study of rural road safety impacts at
At 06:30 PM 12/11/2004 -0500, Tramsol@...
>Todd you might also comment that regulated speeds can dramatically increase
>The German Transport Miinistry produced Stadtverkher im Wandlung (changing
>traffic in towns?) This included an interesting diagram depicting how at
>speed drivers require a smaller dynamic envelope for safe vehicle spacing,
>and a number of other factors such as lane changing etc show that when
>are reduced with good observance of the limit, road capacity increases, and
>multi-lane roads an additional lane can often be added.
>The UK has also delivered this in a practical form on the M25 where
>variable speed limits have kept traffic moving and improved the flow rates
>geting all vehicles conforming to the same speed profile.
>A similar detail can, I believe deliver some interesting potential for bus
>services at a regional centre. By terminating local services from the
>hinterland at peripheral Park & Ride sites and changing bus (the P&R site
>almost instant connection in to town and waiting facilities for the local
>service outbound) Then the service pattern of buses travelling along a
>corridor becomes standardised in vehicle type and stopping pattern and low
>factor rural services no longer require road space, and more important bus
>stances at the journey's end. We are seriously in need of this type of
>in Glasgow, where the streets are clogged with half full buses which either
>run-in or pass through the centre on 2-4 core streets. Many travellers
>gain from outer cross-river services which simply shuttled between the core
>corridors North & South of the river - MVR 1 or 2 vehicles with transfer
>as it is the core soaks up vehicles through the delays in getting through
>But that is slightly OT uless you note that with a 20mph blanket speed
>the traffic might actually flow through the city more freely.
Todd Litman, Director
Victoria Transport Policy Institute
"Efficiency - Equity - Clarity"
1250 Rudlin Street
Victoria, BC, V8V 3R7, Canada
Phone & Fax: 250-360-1560