problem with update from subselect

Hi all, using the schema described below, I want to be able to update each 
arrival time from departure times and trip lengths.

However the update fails because the subselect returns all three answers.

How would I correct the update to make it work

update trip set trip_arrive = (select t.trip_depart + r.route_time from route r, trip t where r.routeid =
t.trip_route);

Database definition:
drop table route;
drop table trip;
drop sequence route_id_seq;
drop sequence trip_id_seq;

create sequence "route_id_seq" start 1 increment 1;
create sequence "trip_id_seq" start 1 increment 1;
create table route ( routeid int4 unique default nextval('route_id_seq'::text) not null, route_depart character (4),
--std ICAO code e.g. EGNM route_dest   character (4),  -- ditto route_time   interval, primary key (routeid)
 
);

create table trip ( tripid int4 unique default nextval('trip_id_seq'::text) not null, trip_route int4 references
route(routeid),trip_depart timestamp,  -- departure time trip_arrive timestamp,  -- calculated ETA primary key
(tripid)
);

insert into route values (1, 'EGNM', 'EGLL', '1 hour 40 minutes');
insert into route values (2, 'EGLL', 'EGKK', '30 minutes');
insert into trip values (1, 1, '2002-01-01 10:00:00');
insert into trip values (2, 1, '2002-02-01 11:30:00');
insert into trip values (3, 2, '2002-01-01 11:00:00');


-- 
Gary Stainburn
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